System crashes! Please help...

I have been just weathering this problem for a while... And I'm so sick of it. I need to know if there's some sensible way I can attack the problem, maybe get some idea of what's going on, short of just replacing all my hardware.

Damn thing crashes. Every now and then it'll just freeze up. Mouse won't respond, virtual terminal switching keystrokes won't respond, can't SSH in, just suddenly, boom, it's done computing things. I don't have a reliable way of triggering it, I have no idea what's causing it and no real idea of how to fix it... Short of, like I said, replacing the damn thing.

I've tried testing the memory with memtest86, stress-testing the CPU with cpuburn to see if I could trigger a crash - got me nowhere. I also tried looking at the logfiles to see if anything weird happened - I haven't managed to find anything.

So if you've got some useful advice on how to diagnose or fix the problem, please, help me out...

We need a little bit more info. Post your Machine's specs here. Which distro are you using? Did you upgrade/update recently? Which Graphics Card do you have?

Hi, thanks for replying. I appreciate any help you can provide.

The systems a few years old now. I think I've had this problem with this hardware since I got it. I upgraded the kernel at one point (and replaced the video driver at the same time, since it's an ATI card and official support for it has been terminated) in the hopes that this would solve the issue.

CPU: Intel(R) Core(TM)2 Quad CPU Q9550 @ 2.83GHz
4GB RAM

lspci output (brief):
00:00.0 Host bridge: Intel Corporation 4 Series Chipset DRAM Controller (rev 02)
00:01.0 PCI bridge: Intel Corporation 4 Series Chipset PCI Express Root Port (rev 02)
00:1a.0 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #4
00:1a.1 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #5
00:1a.2 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #6
00:1a.7 USB controller: Intel Corporation 82801JI (ICH10 Family) USB2 EHCI Controller #2
00:1b.0 Audio device: Intel Corporation 82801JI (ICH10 Family) HD Audio Controller
00:1c.0 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Root Port 1
00:1c.1 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Port 2
00:1d.0 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #1
00:1d.1 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #2
00:1d.2 USB controller: Intel Corporation 82801JI (ICH10 Family) USB UHCI Controller #3
00:1d.7 USB controller: Intel Corporation 82801JI (ICH10 Family) USB2 EHCI Controller #1
00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
00:1f.0 ISA bridge: Intel Corporation 82801JIR (ICH10R) LPC Interface Controller
00:1f.2 IDE interface: Intel Corporation 82801JI (ICH10 Family) 4 port SATA IDE Controller #1
00:1f.3 SMBus: Intel Corporation 82801JI (ICH10 Family) SMBus Controller
00:1f.5 IDE interface: Intel Corporation 82801JI (ICH10 Family) 2 port SATA IDE Controller #2
01:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI RV730XT [Radeon HD 4670]
01:00.1 Audio device: Advanced Micro Devices [AMD] nee ATI RV710/730 HDMI Audio [Radeon HD 4000 series]
03:00.0 SATA controller: Marvell Technology Group Ltd. 88SE9120 SATA 6Gb/s Controller (rev 12)
03:00.1 IDE interface: Marvell Technology Group Ltd. 88SE9128 IDE Controller (rev 12)
04:00.0 FireWire (IEEE 1394): LSI Corporation FW322/323 [TrueFire] 1394a Controller (rev 70)
04:02.0 Ethernet controller: Intel Corporation 82541PI Gigabit Ethernet Controller (rev 05)

Running Debian "testing" branch, usually in X running Gnome. Currently running Linux 3.4.4, previously I was running 2.6.39 - 64 bit kernel running 64 bit userspace

Yesterday the machine crashed, I rebooted and restarted GDM, and it crashed again before showing the login window.
I used to think the problem was triggered by video playback (it would often happen while running VLC) and I thought the video card or its drivers (ATI fire GL drivers, at the time) might be to blame - but I'm currently running the open source driver (Gallium 3D, and xserver-xorg-video-radeon, I guess?) so that possibility seems a bit less likely now.

If there is other information that would help, I'd be happy to provide it. The really frustrating thing about this problem is I don't know how to trigger it, so I can't tell if the problem is fixed...