Ok so i remembered an old problem when right now i'm facing a new one.
2 years ago when i was playing cod4 sometimes the pc would shut down and a tırrrrrr sound would come from the speaker. I had a 8800gt. After suspecting the 400w fsp psu was not enough for the system or dying , i got a new psu back then which was 600W OCZ Stealthstream. And after buying that my pc never shut down even playing a game at %100 load for hours.
NOW: The history repeats itself
Now instead of 8800gt , my gtx 560 started to overheat and again i suspect the PSU (600W OCZ Stealthstream) is not enough to handle 16gb ddr3 ram , i5 2400k , 1tb sata 3 hdd and 200gb sata 2 hdd. Or it is eighter dying.
My suspecion also increases about a thing happened 2 weeks ago. When pc was heavily on use by Unreal Engine 3 , the windows gently shut itself down , and when i restarted it bios said me to reverse the cpu overclock ( Which was 3.2 from 3.1 , that i did 6 months ago!)
How are you checking your temps and what are the temps when your problem happens? Are all of your drivers all up to date? When was the last time you took a can of air to clean all the dust out of the case?
Sorry I had to ask you would be suprised on how many people don't do that. It is not you PSU this time. Can you check your GPU and make sure your fan on it is going?
And make sure all your over clocking is back to normal. You could just be having unstable hard wear by OC'ing. Let us know what say you. Good luck.