Computer restarts by itself

Hello,
My problem is that everytime i get into certain games my computer turns itself off unexpectedly. My specs are:

Intel Q9550
6 Gig RAM
ASUS P5Q Motherboard
NVidia gtx 260
600w PSU
2 Chassis fans

In some games it works for a bit then restarts and others restart when i get into them. When it restarts the fans don't stop moving they just keep going so i don't think it's a PSU problem. I have run speedfan and whatnot, and it's not overheating. I did a memory test and it didn't seem to find anything. I'm really stumped, all i can think of is that the graphics card is faulty or something.
Please reply soon anyone!!