A few weeks ago my pro would not turn on. If I unplugged it and then plugged it back in it would power on and then right back off. I replaced the power supply and decided to take the fan out and clean the vent that it blows through, it was really dirty. Replaced the thermal paste while I was at it. The unit works now but will turn off after 20 minutes of playing a more intensive game. I noticed that on games where the fan used to blow like a jet engine, it no longer does. The fan does not change pace at all. There are no error messages, it just shuts off. It will turn back on after several pushes of the power button. I can get it to run longer by removing the case and blowing a couple of fans on it but it will still only last about an hour or so. I replaced the fan just to be sure but still does the same thing.
Is the fan actually spinning? And have you tried replacing the thermal paste?