Based on the heater observation, it sounds like the coolant is stopping circulating intermittently. That could be due to a loose impeller on the water pump, but that's rare, a slipping water pump belt including those that are driven by the back side of the timing belt, and an air pocket by the thermostat.
Thermostats will not open in the presence of hot air. They must be hit with hot liquid. If a head gasket is leaking and allowing combustion gases to get into the cooling system, an air pocket could form by the thermostat and cause it to close. Hot coolant won't circulate to the radiator or to the heater core.
Your mechanic can test for a head gasket leak with a glass cylinder with two chambers partially filled with a special dark blue liquid. He draws air from the radiator while the car is running. If combustion gases are present, the liquid will turn bright yellow.
Sunday, February 26th, 2012 AT 12:57 AM