If you haven't cleaned the dust from the CPU's cooling system, that's where to start. It might mean opening up the laptop but you can try directing stream of compressed air into the vent ports first. If a cloud of dust emerges, there's a good chance you've discovered the source of the overheating. This method might get you by for a while but dust will accumulate on the fan blades and reduce it's efficiency. The only way to fully clean them is to expose them first.