Thanx for your answer, Bryce.

I'm ssh'ing from home now and I see that friday night it happened again (I left the pc logged it and with a regular bunch of apps running just for testing).

I'm attaching Xorg.0.log. Below is also the output of lspci.

Regarding the backtrace, I'll install the  packages with the debug symbols and leave again the pc logged in.

I have a couple of questions, though... since it seems I can't force this to happen other than "waiting it happens during the night (or a long unattended period)".

Should I launch gdb from another computer or will it suffice if I run it from a text console like tty1 or tty2?

Here's `lspci -vvnn`:
00:00.0 Host bridge [0600]: Intel Corporation 82Q35 Express DRAM Controller [8086:29b0] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort+ >SERR- <PERR- INTx-
	Latency: 0
	Capabilities: <access denied>
	Kernel driver in use: agpgart-intel
	Kernel modules: intel-agp

00:02.0 VGA compatible controller [0300]: Intel Corporation 82Q35 Express Integrated Graphics Controller [8086:29b2] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin A routed to IRQ 16
	Region 0: Memory at fea00000 (32-bit, non-prefetchable) [size=512K]
	Region 1: I/O ports at ec90 [size=8]
	Region 2: Memory at d0000000 (32-bit, prefetchable) [size=256M]
	Region 3: Memory at feb00000 (32-bit, non-prefetchable) [size=1M]
	Capabilities: <access denied>

00:02.1 Display controller [0380]: Intel Corporation 82Q35 Express Integrated Graphics Controller [8086:29b3] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Region 0: Memory at fea80000 (32-bit, non-prefetchable) [size=512K]
	Capabilities: <access denied>

00:03.0 Communication controller [0780]: Intel Corporation 82Q35 Express MEI Controller [8086:29b4] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin A routed to IRQ 16
	Region 0: Memory at fedad000 (64-bit, non-prefetchable) [size=16]
	Capabilities: <access denied>
	Kernel driver in use: heci
	Kernel modules: heci

00:03.2 IDE interface [0101]: Intel Corporation 82Q35 Express PT IDER Controller [8086:29b6] (rev 02) (prog-if 85 [Master SecO PriO])
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem- BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Interrupt: pin C routed to IRQ 18
	Region 0: I/O ports at fe80 [size=8]
	Region 1: I/O ports at fe90 [size=4]
	Region 2: I/O ports at fea0 [size=8]
	Region 3: I/O ports at feb0 [size=4]
	Region 4: I/O ports at fef0 [size=16]
	Capabilities: <access denied>

00:03.3 Serial controller [0700]: Intel Corporation 82Q35 Express Serial KT Controller [8086:29b7] (rev 02) (prog-if 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin B routed to IRQ 17
	Region 0: I/O ports at ec98 [size=8]
	Region 1: Memory at fe9da000 (32-bit, non-prefetchable) [size=4K]
	Capabilities: <access denied>
	Kernel driver in use: serial

00:19.0 Ethernet controller [0200]: Intel Corporation 82566DM-2 Gigabit Network Connection [8086:10bd] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx+
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin A routed to IRQ 221
	Region 0: Memory at fe9e0000 (32-bit, non-prefetchable) [size=128K]
	Region 1: Memory at fe9db000 (32-bit, non-prefetchable) [size=4K]
	Region 2: I/O ports at ecc0 [size=32]
	Capabilities: <access denied>
	Kernel driver in use: e1000e
	Kernel modules: e1000e

00:1a.0 USB Controller [0c03]: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #4 [8086:2937] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin A routed to IRQ 16
	Region 4: I/O ports at ff20 [size=32]
	Capabilities: <access denied>
	Kernel driver in use: uhci_hcd
	Kernel modules: uhci-hcd

00:1a.1 USB Controller [0c03]: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #5 [8086:2938] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin B routed to IRQ 17
	Region 4: I/O ports at ff00 [size=32]
	Capabilities: <access denied>
	Kernel driver in use: uhci_hcd
	Kernel modules: uhci-hcd

00:1a.7 USB Controller [0c03]: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #2 [8086:293c] (rev 02) (prog-if 20)
	Subsystem: Dell Device [1028:0211]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin C routed to IRQ 22
	Region 0: Memory at fe9d9c00 (32-bit, non-prefetchable) [size=1K]
	Capabilities: <access denied>
	Kernel driver in use: ehci_hcd
	Kernel modules: ehci-hcd

00:1b.0 Audio device [0403]: Intel Corporation 82801I (ICH9 Family) HD Audio Controller [8086:293e] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0, Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 16
	Region 0: Memory at fe9dc000 (64-bit, non-prefetchable) [size=16K]
	Capabilities: <access denied>
	Kernel driver in use: HDA Intel
	Kernel modules: snd-hda-intel

00:1c.0 PCI bridge [0604]: Intel Corporation 82801I (ICH9 Family) PCI Express Port 1 [8086:2940] (rev 02)
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx+
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0, Cache Line Size: 64 bytes
	Bus: primary=00, secondary=01, subordinate=01, 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-
		PriDiscTmr- SecDiscTmr- DiscTmrStat- DiscTmrSERREn-
	Capabilities: <access denied>
	Kernel driver in use: pcieport-driver
	Kernel modules: shpchp

00:1d.0 USB Controller [0c03]: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #1 [8086:2934] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin A routed to IRQ 23
	Region 4: I/O ports at ff80 [size=32]
	Capabilities: <access denied>
	Kernel driver in use: uhci_hcd
	Kernel modules: uhci-hcd

00:1d.1 USB Controller [0c03]: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #2 [8086:2935] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin B routed to IRQ 17
	Region 4: I/O ports at ff60 [size=32]
	Capabilities: <access denied>
	Kernel driver in use: uhci_hcd
	Kernel modules: uhci-hcd

00:1d.2 USB Controller [0c03]: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #3 [8086:2936] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem- BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin C routed to IRQ 18
	Region 4: I/O ports at ff40 [size=32]
	Capabilities: <access denied>
	Kernel driver in use: uhci_hcd
	Kernel modules: uhci-hcd

00:1d.7 USB Controller [0c03]: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #1 [8086:293a] (rev 02) (prog-if 20)
	Subsystem: Dell Device [1028:0211]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin A routed to IRQ 23
	Region 0: Memory at ff980800 (32-bit, non-prefetchable) [size=1K]
	Capabilities: <access denied>
	Kernel driver in use: ehci_hcd
	Kernel modules: ehci-hcd

00:1e.0 PCI bridge [0604]: Intel Corporation 82801 PCI Bridge [8086:244e] (rev 92) (prog-if 01)
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Bus: primary=00, secondary=02, subordinate=02, sec-latency=32
	Secondary status: 66MHz- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort+ <SERR- <PERR-
	BridgeCtl: Parity- SERR+ NoISA- VGA- MAbort- >Reset- FastB2B-
		PriDiscTmr- SecDiscTmr- DiscTmrStat- DiscTmrSERREn-
	Capabilities: <access denied>

00:1f.0 ISA bridge [0601]: Intel Corporation 82801IO (ICH9DO) LPC Interface Controller [8086:2914] (rev 02)
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Capabilities: <access denied>
	Kernel modules: iTCO_wdt

00:1f.2 SATA controller [0106]: Intel Corporation 82801IR/IO/IH (ICH9R/DO/DH) 6 port SATA AHCI Controller [8086:2922] (rev 02) (prog-if 01)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
	Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0
	Interrupt: pin C routed to IRQ 222
	Region 0: I/O ports at fe00 [size=8]
	Region 1: I/O ports at fe10 [size=4]
	Region 2: I/O ports at fe20 [size=8]
	Region 3: I/O ports at fe30 [size=4]
	Region 4: I/O ports at fec0 [size=32]
	Region 5: Memory at ff970000 (32-bit, non-prefetchable) [size=2K]
	Capabilities: <access denied>
	Kernel driver in use: ahci
	Kernel modules: ahci

00:1f.3 SMBus [0c05]: Intel Corporation 82801I (ICH9 Family) SMBus Controller [8086:2930] (rev 02)
	Subsystem: Dell Device [1028:0211]
	Control: I/O+ Mem+ BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap- 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Interrupt: pin C routed to IRQ 9
	Region 0: Memory at fe9d9b00 (64-bit, non-prefetchable) [size=256]
	Region 4: I/O ports at ece0 [size=32]
	Kernel modules: i2c-i801