Yeah, my thought was video/graphics card. Taking the card out and putting it back in again often sorts out those sort of problems. (Obviously do that when it's switched off & unplugged!)
It could be overheating, I guess. Does it always go "whack" after it's been switched on approx the same amount of time? (that might suggest overheating), or does it just to it randomly. Bear in mind that if it's overheating, the length of time it takes to happen would be different depending on if it's been switched off for a few hours or if you just rebooted after the last crash, because then it'd still be quite warm so would crash faster the second time.