Darn it Fred, why do you have your PMs turned off? Anyway, what you are describing sounds more like a software problem than hardware to me. I have had a power supply go bad and the computer just stops, it doesn't go through the shutdown routine. Also I am sure that you have cleaned the inside and looked to see that the fan is working. Just how old is this computer? When you re-loaded it, did you format the drive or just do the re-load?
Ron