Could be an air pocket in the cooling system. Open the radiator cap (when the engine is cold), not the oveflow tank, and check that it really is full with coolant.
When the engine is cold, open the radiator cap (the one with the spring on it, in the middle of the radiator, not the plastic cap on the overflow tank). Check to see that it is full to the very brim with coolant (water plus anti-freeze, in a 50-50 mix). If it isn't full, fill it right up, then start the engine, let it idle, and watch the level. If the level drops, add more coolant, then close the cap before the engine really warms up (or the coolant will start to overflow).atoyotchick said:How does one go about getting air out of the system, if that is the problem? Thanks