I had a similar problem with random reboots and tried EVERYTHING it drove me C R A Z Y. OCUK even suggested purchasing a new PSU, which i did.
Reboots continued. Then tried a new motherboard - reboots continued.
Turned out to be my graphics card. Upgraded to an 8800GTS and problem solved.
Previously I could get between 5 and 50 mins use out of my PC before it rebooted. Now it's stable 24/7.
If you could borrow another card for a couple of days you may find better results.