Same issue here, entirely different sounding hardware. Running NetworkManager. Your symptoms are identical to mine. I have not found any pattern here, although increased network usage sounds like a good guess. Running a system monitor applet on my panel set to measure system load average, and have observed on a few occasions the load jumping off the chart (13 or so!) before the system entirely freezes. Usually freezes about a second after the load spikes upwards. Unfortunately, I have not been able to confirm this consistently since the crash is so inconsistent itself.

Alt+SysRQ+1 then Alt+SysRQ+t do nothing. Mind you, my log is being spammed with an error message about wireless encryption (thanks, Netgear!), but I am fairly certain it achieved nothing. (Feel free to show otherwise).

# lspci -v

00:00.0 Host bridge: Intel Corporation Mobile PM965/GM965/GL960 Memory Controller Hub (rev 03)
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, fast devsel, latency 0
	Capabilities: <access denied>

00:01.0 PCI bridge: Intel Corporation Mobile PM965/GM965/GL960 PCI Express Root Port (rev 03) (prog-if 00 [Normal decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=01, subordinate=01, sec-latency=0
	I/O behind bridge: 00009000-00009fff
	Memory behind bridge: f7e00000-fdefffff
	Prefetchable memory behind bridge: 00000000bdf00000-00000000ddefffff
	Capabilities: <access denied>

00:1a.0 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Contoller #4 (rev 03) (prog-if 00 [UHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 16
	I/O ports at e080 [size=32]

00:1a.1 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #5 (rev 03) (prog-if 00 [UHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 21
	I/O ports at e000 [size=32]

00:1a.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #2 (rev 03) (prog-if 20 [EHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 18
	Memory at febff400 (32-bit, non-prefetchable) [size=1K]
	Capabilities: <access denied>

00:1b.0 Audio device: Intel Corporation 82801H (ICH8 Family) HD Audio Controller (rev 03)
	Subsystem: ASUSTeK Computer Inc. Unknown device 1339
	Flags: bus master, fast devsel, latency 0, IRQ 22
	Memory at febf8000 (64-bit, non-prefetchable) [size=16K]
	Capabilities: <access denied>

00:1c.0 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 1 (rev 03) (prog-if 00 [Normal decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=02, subordinate=02, sec-latency=0
	Capabilities: <access denied>

00:1c.1 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 2 (rev 03) (prog-if 00 [Normal decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=03, subordinate=03, sec-latency=0
	Memory behind bridge: fdf00000-fdffffff
	Capabilities: <access denied>

00:1c.2 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 3 (rev 03) (prog-if 00 [Normal decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=04, subordinate=04, sec-latency=0
	I/O behind bridge: 0000a000-0000afff
	Memory behind bridge: fe000000-fe0fffff
	Capabilities: <access denied>

00:1c.3 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 4 (rev 03) (prog-if 00 [Normal decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=05, subordinate=05, sec-latency=0
	I/O behind bridge: 0000b000-0000bfff
	Memory behind bridge: fe100000-fe1fffff
	Capabilities: <access denied>

00:1c.4 PCI bridge: Intel Corporation 82801H (ICH8 Family) PCI Express Port 5 (rev 03) (prog-if 00 [Normal decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=06, subordinate=07, sec-latency=0
	I/O behind bridge: 0000c000-0000cfff
	Memory behind bridge: fe200000-fe9fffff
	Prefetchable memory behind bridge: 00000000ddf00000-00000000dfefffff
	Capabilities: <access denied>

00:1d.0 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #1 (rev 03) (prog-if 00 [UHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 23
	I/O ports at dc00 [size=32]

00:1d.1 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #2 (rev 03) (prog-if 00 [UHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 19
	I/O ports at d880 [size=32]

00:1d.2 USB Controller: Intel Corporation 82801H (ICH8 Family) USB UHCI Controller #3 (rev 03) (prog-if 00 [UHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 18
	I/O ports at d800 [size=32]

00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 23
	Memory at febff000 (32-bit, non-prefetchable) [size=1K]
	Capabilities: <access denied>

00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev f3) (prog-if 01 [Subtractive decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=08, subordinate=08, sec-latency=32
	Memory behind bridge: fea00000-feafffff
	Capabilities: <access denied>

00:1f.0 ISA bridge: Intel Corporation 82801HEM (ICH8M) LPC Interface Controller (rev 03)
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0
	Capabilities: <access denied>

00:1f.1 IDE interface: Intel Corporation 82801HBM/HEM (ICH8M/ICH8M-E) IDE Controller (rev 03) (prog-if 8a [Master SecP PriP])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 0, IRQ 18
	I/O ports at 01f0 [size=8]
	I/O ports at 03f4 [size=1]
	I/O ports at 0170 [size=8]
	I/O ports at 0374 [size=1]
	I/O ports at ffa0 [size=16]

00:1f.2 SATA controller: Intel Corporation 82801HBM/HEM (ICH8M/ICH8M-E) SATA AHCI Controller (rev 03) (prog-if 01 [AHCI 1.0])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, 66MHz, medium devsel, latency 0, IRQ 504
	I/O ports at ec00 [size=8]
	I/O ports at e880 [size=4]
	I/O ports at e800 [size=8]
	I/O ports at e480 [size=4]
	I/O ports at e400 [size=32]
	Memory at febff800 (32-bit, non-prefetchable) [size=2K]
	Capabilities: <access denied>

01:00.0 VGA compatible controller: nVidia Corporation GeForce 8600M GT (rev a1) (prog-if 00 [VGA controller])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1632
	Flags: bus master, fast devsel, latency 0, IRQ 16
	Memory at fc000000 (32-bit, non-prefetchable) [size=16M]
	Memory at c0000000 (64-bit, prefetchable) [size=256M]
	Memory at fa000000 (64-bit, non-prefetchable) [size=32M]
	I/O ports at 9c00 [size=128]
	[virtual] Expansion ROM at fdee0000 [disabled] [size=128K]
	Capabilities: <access denied>

03:00.0 Network controller: Intel Corporation PRO/Wireless 4965 AG or AGN Network Connection (rev 61)
	Subsystem: Intel Corporation Unknown device 1100
	Flags: bus master, fast devsel, latency 0, IRQ 503
	Memory at fdffe000 (64-bit, non-prefetchable) [size=8K]
	Capabilities: <access denied>

04:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 01)
	Subsystem: ASUSTeK Computer Inc. Unknown device 11f5
	Flags: bus master, fast devsel, latency 0, IRQ 505
	I/O ports at a800 [size=256]
	Memory at fe0ff000 (64-bit, non-prefetchable) [size=4K]
	Expansion ROM at fe0c0000 [disabled] [size=128K]
	Capabilities: <access denied>

05:00.0 Memory controller: Intel Corporation Turbo Memory Controller (rev 01)
	Subsystem: Intel Corporation Turbo Memory Controller
	Flags: bus master, fast devsel, latency 0, IRQ 3
	Memory at fe1ffc00 (32-bit, non-prefetchable) [size=1K]
	I/O ports at bc00 [size=128]
	Expansion ROM at fe1e0000 [disabled] [size=64K]
	Capabilities: <access denied>

08:03.0 FireWire (IEEE 1394): Ricoh Co Ltd R5C832 IEEE 1394 Controller (rev 05) (prog-if 10 [OHCI])
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 64, IRQ 21
	Memory at feaff800 (32-bit, non-prefetchable) [size=2K]
	Capabilities: <access denied>

08:03.1 SD Host controller: Ricoh Co Ltd R5C822 SD/SDIO/MMC/MS/MSPro Host Adapter (rev 22)
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 64, IRQ 20
	Memory at feaff400 (32-bit, non-prefetchable) [size=256]
	Capabilities: <access denied>

08:03.2 System peripheral: Ricoh Co Ltd R5C592 Memory Stick Bus Host Adapter (rev 12)
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 64, IRQ 6
	Memory at feaff000 (32-bit, non-prefetchable) [size=256]
	Capabilities: <access denied>

08:03.3 System peripheral: Ricoh Co Ltd xD-Picture Card Controller (rev 12)
	Subsystem: ASUSTeK Computer Inc. Unknown device 1517
	Flags: bus master, medium devsel, latency 64, IRQ 6
	Memory at feafec00 (32-bit, non-prefetchable) [size=256]
	Capabilities: <access denied>



My hardware, in basic terms:
Asus F8SV-A1
Intel Core 2 Duo T7500 @ 2.20 GHz
2 GB RAM
NVidia 8600M GT
Intel 4965AGN WiFi adaptor (runs out of the box with included drivers)