I doubt very much that it is a head gasket, I wouldn't worry about that. As I said, my car was doing this a lot and I never bothered about it. Later, I broke a piston and the engine was rebuilt using the same block but with replacement forged-pistons. Before rebuilding the engine, the car went through extensive tests in order to find the fault (you wouldn't jsut pull an engine apart, would you?) and at no point was the head gasket diagnosed as being at fault. I have read from other posts that the most likely casue of this is in fact the heater matrix.