If you get 60+ FPS after starting the game but then lose FPS untill you get intermittent drops to less than 5 FPS, there is a good chance that either your CPU or GPU is overheating and has started to throttle its performance. You can try to fiddle with softwares and alter settings in the BIOS to reduce this issue, but this here might do the trick in a more reliable and less risky way (as far as hardware is concerned). Get one of these: http://en.wikipedia.org/wiki/Laptop_cooler Since getting one I get consistent FPS.
