I have a 2006 6 cylinder Gran Prix. With the coolant full I start out and the heater and temp gauge work fine. After about 1 hour of drive time the heater starts blowing cool air and the temp gauge goes up. I park and check the engine compartment and there is coolant leakage around the belts and back engine area. After the car sits for some time at work I fill the coolant and the process repeats itself. Could this be a thermostat problem and would it purge coolant if this is the case?