[Monkey]

Have you checked the power supply. If you have a 300 Watt Power Supply, I would upgrade to a beefy 400 Watt with dual fans. You may want to check your power supply with a voltmeter.

 

[Monkey]

If you have an extra power supply sitting around I would try that. The other option might be memory. If you have 2 sticks of memory, I would remove one, see if you still have the problem. If you still do, swap with the other. When It comes to the computer restarting itself, I hear that the two main problems are either the power supply or memory.