Sounds like the CPU may be overheating. If you are confident enough, open your PC (Removing power first of course) locate your CPU on the motherboard, and have a look at the cooling fan, It and the heat sink may be clogged with dust. If you are confident enough, remove the cooling fan, and while using a small brush, vaccum all the dust out. Clean out the rest of the case while you are at it.
Now power the puter back up and see how it goes. While the side is off, direct the blast from a small electric fan into the case, this should improve the cooling (I do this in summer here in West Oz.
Best of luck.
