The button is most likely going out. Either you can get a new case and transfer everything over to it( very drastic and might not work if its an oem computer from a store) or you can buy a push button switch from a hardware store and drill a hole in the front of the case and hook it up. I actually had to do that to an HP computer due to some kids not knowing what they were doing and ransacked someone pc for parts, which I had to end up fixing back up.