Computer is overheating. Need to replace the thermal paste in the cpu. A couple of dollars to buy this at radio shack and your good as new. FYI, I build computers as a side job, this sounds like the problem your having. It starts out where your don't have problems for a while but as the paste degrades, time spent on the computer decreases and usually there's no warning besides computer turning off. If this was a power problem, it would show at start up as not all systems would work.