|Could be overheating, cooling down when there is little activity. Did you clean the CPU heat sink as well as the vents? Check it with the sides are in place because when they are off they can cause a poor ventilation flow.|
How long does the computer have to be on before it happens?
If it has sensors you should be able to get the temperatures from BIOS or by downloading a free program.
Always pop back and let us know the outcome - thanks