A lot of machines have a threshold setting in the BIOS - if the CPU or system temperature reaches the threshold it will power down. A lot also have a diagnostic 'beep' system that can give you an idea of the problem - eg. on a HP machine 5 beeps and a pause means the RAM is the problem. If you have something like that going on, look for the motherboard model and google it or dig out your manual. A common problem I had with overheating motherboards was the chipset (usually has a smaller heatsink than the CPU). Removing the heatsink and fitting a better one, or cleaning and repapplying it with new thermal paste often helps. First thing to do though is open it up and blast everything with an air-duster. Also check everything is still connected. Make sure the fans still spin, too.
Good luck.
Last edited by Bluebird007 on Fri Aug 13, 2010 4:25 pm, edited 1 time in total.