Crash and restart?

I am helping a friend who knows even less than me about computers. So here is the story. He had no video so I brought his computer home and proceeded to check it out. I determined that his mainboard was shot. I replaced the mainboard and everything was testing fine when the computer just shut down and restarted. I checked for a virus and loaded Ca internet security suite, found nothing wrong. It seems that is shuts down after about 30 minutes the first time. Then after it is warmed up it is more frequent. I will change the processor tonight and see if that helps. Do you have any ideas?

 

I vote power supply. If you don't have a PS tester they are only about $12. This exact thing was happening to me and I replaced it and it ran perfect.

 

I guess you were correct. When I removed the fan and heat sync from the processor the thermal grease was all dried out and there was none on the processor. I cleaned up both and applied new thermal grease and it has been running now for several hours just fine. Thanks all.

 

Thermal paste was dried up??? wow, that computer must be old... not implying anything oldsalty

 

Yes it is. Or it is a very hot processor. I am not sure but it is about 6-7 years old. eMachine T2200