it sounds as if it is overheating. see if you can take off the bottom cover to expose any fans/heatsinks and give them a blow out with a can of compressed air. If you can't lay your hands on a can of air, brush out with a small paintbrush. it is quite normal for laptops to get heatsinks that are "congealed" with crud from being sat on your legs or the floor as it will pick up dust.