It could be a zillion thing! but the psu is always the usual suspect, graphic cards requires 550 watt or above..i'm assuming you've got that covered?
mine requires 550, but i installed a 650 to be on the safe side..
also it might be helpful to check your event viewer for errors..
I remember once my system started shutting down for no reason whatsoever!! I went crazy and it turned out that a software i previously uninstalled 'called online armor' was the reason!
anyway, check your event viewer..and next time when it happened, remember what application/s was running before the crash.
good luck.