So it is either a CPU overheating or your PSU is going out. So download this: http://www.techpowerup.com/downloads/1872/Real_Temp_3.60.html
(download button is on the right). Once downloaded, run it. It won't install anything. It will display your processor temperatures. If you minimize it, the temperature will read on the bottom right from the task bar. Keep an eye on it, and next time it shuts down write what the last temp was that you saw. Also, post back here with what the temperature reads normally.
By the way, there is an ad in my post because this forum automatically does this. It is not in my control. I don't sell anything. Many websites do this sort of advertising, taking select words and turning them into a link to an ad.