A good rule of thumb is never to buy a used motherboard if you can help it, because you never know what kind of abuse it had been subjected to. A new mobo doesn't cost much more than a used one. Here are some possibilities. Motherboard may be bad, or the on-off switch on your pc may be bad. Next time this happens again, unplug the power supply and open the case, touch the heat sink on top of the processor to see if it is overheated. You may have installed the processor incorrectly, or the processor may be wrong for your motherboard.
