James, there are several factors that you have mentioned that could be causing your problem. First and foremost it could be a heat issue and lack of air circulation inside of the computer beneath the logic board. If you have access to a small air compressor, close the lid and look for the air vents along the back edge of the laptop. Use the compressor to force air through the back side of the laptop. This should open the air vent grills inside of the computer to allow cooling air to pass through. If this does not work, then you need to remove the logic board and clean all of the dust out, in particular the the grill of the heatsink bracket. Once done then clean the heatsink compound of both of the heat sink pads and also the logig board and replace it with new Arctic Silver Heatsink compound. If you have already done this and you are still having a problem, I would then seriously suspect bad RAM chips. These will go bad and cause all of the same issues. Replace them one at a time with known good ones. I'm sure it is not the logic board as it is still functioning.
ED