It could be several things. If you were actually overheating I would expect you would have other signs, such as puking water, blowing steam, detonation, etc.
It could be a fuel delivery problem. If you have a fuel filter change it. Inspect the gas hose between the tank and the carburetor to make sure it doesn't have a kink in it. Try taking the fuel line off at the carburetor and then opening the gas valve at the tank (be ready to catch gas in a jar or something). You should get a good steady flow of gas. If the gas dribbles out then it probably isn't flowing enough gas. The float bowl will fill up when you are idle, but after a few laps the reservoir in the float bowl gets used up.
Another thing you might want to try is to re-torque the head down. It is possible that the head is a little too loose and when it heats up the head gasket leaks.
Rod