Computer shuts down after some minutes

[Gharren]

I had the exact same problem about two years ago, but I'm not sure how I solved it. xD
But I'd recommend you to do something about it asap because overheating massively reduces the lifetime of your CPU and other components.

First, check if the fan runs properly. Does it get very loud/increase RPM if you play games, as it should if the CPU gets too hot? If yes, then theres probably something wrong with the thermal paste and you should replace it. I can recommend you "Arctic Cooling MX-2". It's cheap, easy to apply and works fine for me.