what temps is your hardware reaching when you get the slowdown and what voltages? Have you scanned your computer with Malware, spyware, and virus detecting software? what power supply and ROM Drives do you have? have you checked the processes running when you computer is slowing down?
It could also easily be a trojan or virus, that is being a resource hog.
If he has installed his OS on another harddrive and installed every again, then there is a one in a Billion chance that a virus or Trojan would still be there, and if it was then it would have done something by now to completely kill some if not all of the hardware.