First off, I'd attempt to eliminate hardware as a source of the problems before spending a lot of time reinstalling. Have you opened it up, reseated all the cables, cards, memory, and the like? Have you checked to see that the processor fan is running, and the heatsink is properly attached? If it has a system monitor utility, check the operating temperatures of the processor and see if it's in the 40's idling, any more and that's a potential problem.