I have this problem at times too. I don't know that it's PSU related.
My PC spec i2600k, EVGA GTX 570, 8gig Ram 1600mhz, Seasonic 750Watt 80plus, 1 bluray drive, 1 80gig SSD drive, 1tb WD black drive.
Power should be enough and Seasonic is a damn good brand PSU. I'm not sure why my PC still turns off at time when I play graphic heavy game.