Most of the time when a Mac overheats it does an auto-shutdown for self protection. I would open it, see if the fan is running, remove the fan and clean the vent, then test again. To help us with diagnostics please give us the last four figures of your serial number. To test for a software conflict or possible virus, hold down the Shift key on start up. Let us know if any of your results change. Also how much available free hard drive space do you have and what OS are you running?