3.5 water pumps are known to fail and create the same condition. If you are sure it is an overheat problem and not a gauge problem I would replace the water pump.
Unless you have replaced on a 3.5 at least 3 times successfully don't trty it. It is driven by the timing belt and it is an interference engine. Off ine tooth and you have wiped the valve train. Let a pro do it. Good luck