Computer Randomly Restarts When Playing Games

When I first got my computer, it worked just fine, I played all my games such as Call of Duty Modern Warfare 2 and WoW on the fullest settings without a problem. Then a few months later it began restarting after anywhere between 30 mins to 1hour+ of playing. I called the makers of my computer, we tried to see if it was overheating but it wasnt, I also tested the videocards, it wasnt that either. Then, out of nowhere it fixed itself for a few months, then out of nowhere it happened again. I restored my computer to an earlier time and it worked for a few days and now it's restarting again.