My PC self-reboots: Faulty Motherboard
If all else fails and your PC still spontaneously reboots itself on a random basis, then there is only one thing more that can cause it - Big Mama board is busted. This is now the right time to replace the poor motherboard. But as said in the second post, it would be better if you have the motherboard replaced at your trusted repair shop. And while you are at it, (again as mentioned in the same post), you should also better replace the PSU to save you time and effort in the future.
Source: Answers that Work