Have you changed your coolant recently? If not, it may be time to get some new fluid in the car. Also, check the radiator hoses for any kinks or twists that may cut off coolant flow. Make sure there's no air in the coolant system ... here's how to "bleed" it. After the car has sat overnight, take off the radiator cap and start the car. Turn the heat and fan on high. Let car run until the radiator fan has come on twice. If there's any air in the system, it will work it's way out and come to the top of the radiator.