This also does not happen every time. I currently work with the external display with correct settings. I think it mainly happens when I disable the notebook display AND enable the external display with its native resolution on the same time. 

johannes@jupiter:~$ xrandr
Screen 0: minimum 320 x 200, current 1680 x 1050, maximum 1920 x 1200
VGA disconnected (normal left inverted right x axis y axis)
LVDS connected (normal left inverted right x axis y axis)
   1440x900       60.0 +   50.0  
   1280x800       60.0  
   1280x768       60.0  
   1024x768       60.0  
   800x600        60.3  
   640x480        59.9  
TMDS-1 connected 1680x1050+0+0 (normal left inverted right x axis y axis) 434mm x 270mm
   1680x1050      59.9*+
   1280x1024      75.0     59.9  
   1152x864       74.8  
   1024x768       75.1     60.0  
   800x600        75.0     60.3  
   640x480        75.0     60.0  
   720x400        70.1  
TV disconnected (normal left inverted right x axis y axis)




johannes@jupiter:~$ lspci -vvnn
00:00.0 Host bridge [0600]: Intel Corporation Mobile PM965/GM965/GL960 Memory Controller Hub [8086:2a00] (rev 0c)
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort+ >SERR- <PERR-
	Latency: 0
	Capabilities: <access denied>

00:02.0 VGA compatible controller [0300]: Intel Corporation Mobile GM965/GL960 Integrated Graphics Controller [8086:2a02] (rev 0c) (prog-if 00 [VGA controller])
	Subsystem: Dell Latitude D630 [1028:01f9]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin A routed to IRQ 16
	Region 0: Memory at fea00000 (64-bit, non-prefetchable) [size=1M]
	Region 2: Memory at e0000000 (64-bit, prefetchable) [size=256M]
	Region 4: I/O ports at efe8 [size=8]
	Capabilities: <access denied>

00:02.1 Display controller [0380]: Intel Corporation Mobile GM965/GL960 Integrated Graphics Controller [8086:2a03] (rev 0c)
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Region 0: Memory at feb00000 (64-bit, non-prefetchable) [size=1M]
	Capabilities: <access denied>

00:1a.0 USB Controller [0c03]: Intel Corporation 82801H (ICH8 Family) USB UHCI Contoller #4 [8086:2834] (rev 02) (prog-if 00 [UHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin A routed to IRQ 20
	Region 4: I/O ports at 6f20 [size=32]

00:1a.1 USB Controller [0c03]: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #5 [8086:2835] (rev 02) (prog-if 00 [UHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin B routed to IRQ 21
	Region 4: I/O ports at 6f00 [size=32]

00:1a.7 USB Controller [0c03]: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #2 [8086:283a] (rev 02) (prog-if 20 [EHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin C routed to IRQ 22
	Region 0: Memory at fed1c400 (32-bit, non-prefetchable) [size=1K]
	Capabilities: <access denied>

00:1b.0 Audio device [0403]: Intel Corporation 82801H (ICH8 Family) HD Audio Controller [8086:284b] (rev 02)
	Subsystem: Dell Dell Latitude D630 [1028:01f9]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0, Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 21
	Region 0: Memory at fe9fc000 (64-bit, non-prefetchable) [size=16K]
	Capabilities: <access denied>

00:1c.0 PCI bridge [0604]: Intel Corporation 82801H (ICH8 Family) PCI Express Port 1 [8086:283f] (rev 02) (prog-if 00 [Normal decode])
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0, Cache Line Size: 64 bytes
	Bus: primary=00, secondary=0b, subordinate=0b, sec-latency=0
	Secondary status: 66MHz- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort+ <SERR- <PERR-
	BridgeCtl: Parity- SERR+ NoISA- VGA- MAbort- >Reset- FastB2B-
	Capabilities: <access denied>

00:1c.1 PCI bridge [0604]: Intel Corporation 82801H (ICH8 Family) PCI Express Port 2 [8086:2841] (rev 02) (prog-if 00 [Normal decode])
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0, Cache Line Size: 64 bytes
	Bus: primary=00, secondary=0c, subordinate=0c, sec-latency=0
	Memory behind bridge: fe800000-fe8fffff
	Secondary status: 66MHz- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort+ <SERR- <PERR-
	BridgeCtl: Parity- SERR+ NoISA- VGA- MAbort- >Reset- FastB2B-
	Capabilities: <access denied>

00:1c.5 PCI bridge [0604]: Intel Corporation 82801H (ICH8 Family) PCI Express Port 6 [8086:2849] (rev 02) (prog-if 00 [Normal decode])
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0, Cache Line Size: 64 bytes
	Bus: primary=00, secondary=09, subordinate=09, sec-latency=0
	Memory behind bridge: fe700000-fe7fffff
	Secondary status: 66MHz- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort+ <SERR- <PERR-
	BridgeCtl: Parity- SERR+ NoISA- VGA- MAbort- >Reset- FastB2B-
	Capabilities: <access denied>

00:1d.0 USB Controller [0c03]: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #1 [8086:2830] (rev 02) (prog-if 00 [UHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin A routed to IRQ 20
	Region 4: I/O ports at 6f80 [size=32]

00:1d.1 USB Controller [0c03]: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #2 [8086:2831] (rev 02) (prog-if 00 [UHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin B routed to IRQ 21
	Region 4: I/O ports at 6f60 [size=32]

00:1d.2 USB Controller [0c03]: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #3 [8086:2832] (rev 02) (prog-if 00 [UHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin C routed to IRQ 22
	Region 4: I/O ports at 6f40 [size=32]

00:1d.7 USB Controller [0c03]: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 [8086:2836] (rev 02) (prog-if 20 [EHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin A routed to IRQ 20
	Region 0: Memory at fed1c000 (32-bit, non-prefetchable) [size=1K]
	Capabilities: <access denied>

00:1e.0 PCI bridge [0604]: Intel Corporation 82801 Mobile PCI Bridge [8086:2448] (rev f2) (prog-if 01 [Subtractive decode])
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Bus: primary=00, secondary=03, subordinate=07, sec-latency=32
	Memory behind bridge: fe600000-fe6fffff
	Secondary status: 66MHz- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort+ <SERR- <PERR+
	BridgeCtl: Parity- SERR+ NoISA- VGA- MAbort- >Reset- FastB2B-
	Capabilities: <access denied>

00:1f.0 ISA bridge [0601]: Intel Corporation 82801HEM (ICH8M) LPC Interface Controller [8086:2815] (rev 02)
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Capabilities: <access denied>

00:1f.1 IDE interface [0101]: Intel Corporation 82801HBM/HEM (ICH8M/ICH8M-E) IDE Controller [8086:2850] (rev 02) (prog-if 8a [Master SecP PriP])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin A routed to IRQ 16
	Region 0: I/O ports at 01f0 [size=8]
	Region 1: I/O ports at 03f4 [size=1]
	Region 2: I/O ports at 0170 [size=8]
	Region 3: I/O ports at 0374 [size=1]
	Region 4: I/O ports at 6fa0 [size=16]

00:1f.2 IDE interface [0101]: Intel Corporation 82801HBM/HEM (ICH8M/ICH8M-E) SATA IDE Controller [8086:2828] (rev 02) (prog-if 8f [Master SecP SecO PriP PriO])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
	Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0
	Interrupt: pin B routed to IRQ 17
	Region 0: I/O ports at 6eb0 [size=8]
	Region 1: I/O ports at 6eb8 [size=4]
	Region 2: I/O ports at 6ec0 [size=8]
	Region 3: I/O ports at 6ec8 [size=4]
	Region 4: I/O ports at 6ee0 [size=16]
	Region 5: I/O ports at eff0 [size=16]
	Capabilities: <access denied>

00:1f.3 SMBus [0c05]: Intel Corporation 82801H (ICH8 Family) SMBus Controller [8086:283e] (rev 02)
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem+ BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Interrupt: pin B routed to IRQ 10
	Region 0: Memory at fe9fbf00 (32-bit, non-prefetchable) [size=256]
	Region 4: I/O ports at 10c0 [size=32]

03:01.0 CardBus bridge [0607]: O2 Micro, Inc. Cardbus bridge [1217:7135] (rev 21)
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping+ SERR- FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=slow >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 168
	Interrupt: pin A routed to IRQ 19
	Region 0: Memory at fe600000 (32-bit, non-prefetchable) [size=4K]
	Bus: primary=03, secondary=04, subordinate=07, sec-latency=176
	Memory window 0: 88000000-8bfff000 (prefetchable)
	Memory window 1: 8c000000-8ffff000
	I/O window 0: 00001400-000014ff
	I/O window 1: 00001800-000018ff
	BridgeCtl: Parity- SERR- ISA- VGA- MAbort- >Reset+ 16bInt+ PostWrite+
	16-bit legacy interface ports at 0001

03:01.4 FireWire (IEEE 1394) [0c00]: O2 Micro, Inc. Firewire (IEEE 1394) [1217:00f7] (rev 02) (prog-if 10 [OHCI])
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV+ VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 64, Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 19
	Region 0: Memory at fe6ff000 (32-bit, non-prefetchable) [size=4K]
	Region 1: Memory at fe6fe800 (32-bit, non-prefetchable) [size=2K]
	Capabilities: <access denied>

09:00.0 Ethernet controller [0200]: Broadcom Corporation NetXtreme BCM5755M Gigabit Ethernet PCI Express [14e4:1673] (rev 02)
	Subsystem: Dell Unknown device [1028:01f9]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0, Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 507
	Region 0: Memory at fe7f0000 (64-bit, non-prefetchable) [size=64K]
	Capabilities: <access denied>

0c:00.0 Network controller [0280]: Intel Corporation PRO/Wireless 3945ABG Network Connection [8086:4222] (rev 02)
	Subsystem: Intel Corporation Unknown device [8086:1021]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
	Latency: 0, Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 508
	Region 0: Memory at fe8ff000 (32-bit, non-prefetchable) [size=4K]
	Capabilities: <access denied>