PC Crashing... hope you all like a challenge

A few days ago I walked into my living room to find that the monitor had gone into sleep mode, the mouse and keyboard had switched off, and all the fans in my case were absolutely blazing-- most notably, the blower fan on my Radeon HD 5850. Although the components seemed to still be running, Windows or BIOS clearly weren't. Doing a cold shutdown and reboot only returned it to the same "blaring," no-POST state. Waiting about half an hour though seemed to do the trick and the OS started up normally again.

The first thing I thought was overheating, as I'm sure you're thinking, too. Nope!
Here is my computer at idle: imgur.com/G8bM5.png
Here is my computer at full load: imgur.com/JAzx4.jpg (for whatever reason, links aren't appearing when I post)
Stable as a table. When the computer crashes, it isn't anywhere near these temps or specs.

When this first happened, when the computer came back on, all of my USB ports had been disabled. Uninstalling them all from device manager and letting Windows reactivate them seemed to do the trick, as they all work now (I'm glad I held onto my old PS/2 mouse and keyboard, not to mention that little green adapter).

I can use the computer for hours now, but if I let it sit for even just half an hour, it will crash and enter that bizarre state when all the fans are blazing and nothing is running. It seems to only crash when idle.

The latest hardware change is the 5850, which I added a few months ago. It used to be an X1950XTX, which I still have available. I'll probably pop that back in to see if it's the video card causing the issues.

Anyone have any ideas?

Computer specs:
Antec 1200
Gigabyte GA-965P-DS3
Intel Q6600
3x 1GB GSkill DDR2 800
XFX Radeon HD 5850
WD Black 1TB
Seagate 250GB
Maxtor 200GB
BGears b-enspirer
Syba Firewire card
OCZ600GSXSLI 600W PSU

 

Sorry to see this thread go completely forgotten, but for those who find this post via a search, here's how to solve it: update the Catalyst drivers from 10.3 to 10.6 (or latest version). It was such an obvious fix I nearly fell over when I finally remembered it. Since the video card fan was blasting following the reboot every time, I figured that it was probably causing the issue, but the sudden power loss made me think it was bad memory, bad timings, or a bad PSU. Worse yet, a bad mobo or CPU. At no point was the computer overheating.

Hope this helps.

 

"The latest hardware change is the 5850, which I added a few months ago. It used to be an X1950XTX, which I still have available. I'll probably pop that back in to see if it's the video card causing the issues."

You said it, and coming back and showing the answer will always help somebody-