Heating/Cooling Problem

It sounds like a head gasket is blown. Thats a common sign of pressure backing up into the over flow. Take it in and ask for a cooling system pressure test, that will tell you if its the radiator or the head gasket. Once they pressurize the system, it usually pushes antifreeze out of the probelm spot(like the rad) if not... its the head. Also check the Thermostat, it could be stuck in a partial closed position. If the engine is a high mile'er , replace it anyway. they are cheap, around $15.00 or so. DO NOT START REPLACING PARTS!!! spend the money on a "Correct diagnostic" of your problem