I presume you are overclocking your system????
Did returning to stock settings not help?
I have had faulty PSU's before; the computer would not necessarily restart/shut off; sometimes it would just crash back to desktop or freeze like so.
Also, Prime95 stable for 8 hours means... NOTHING. I once had a Prime stable configuration for 15 hours. Started up a game and got a BSOD in 2 seconds.