I have been having alot of trouble with my computer shutting down while gaming recently. The computer ran perfect for about a year and a half and now I cant load any high graphics games without it shutting down and restarting. There are no error messages and I have downloaded several hardware monitoring programs (which didn't help). I have torture tested the computer with prime95 and it does not shut down. The power supply was recently sent back to the company becuase the fan stopped working in it which i noticed when the computer first started this problem. The problem was worse then and probably heat related as the computer would shut down and restart several times in a row after gaming for awhile. After I got the power supply back the computer started just shutting down when trying to load Elder Scrolls Skyrim, and everytime i loaded Battlefield 3 and when I would attempt to do the Rate your performance tool on Windows 7. I recently replaced the graphics card and am still having the exact same problem, these are the components of my computer.
EVGA X58 SLI 3 motherboard
Coolmaster GX 750 power supply
corsair XMS3 12GB triple channel ram
Intel I7 950 quad core processor
Zotac GTX560 2gb GDDR5 video card
WD Caviar Blue 500gb HD
Prime95 tests your cpu/ram. It sounds like your graphics card is overheating. Which temperature programs are you using? I use HWmonitor and it isn't perfect but it does provide graphic card temps. Do you have a side fan that you can use to blow on your graphics card? Also make sure the gpu's exhaust isn't being blocked.