Also try bleeding the cooling system. Look at the top of your water neck on the motor. There should be a bolt/screw that goes into it. This should be the highest point of the cooling system, therefore if air is in the system, it will come here. With the car on level ground, let it run at idle and crack that bleed screw open. Fill up the radiator with water until it comes out of the bleed hole. Also try it with your heater all the way on full blast. Try that and let me know if you still have the problem.