hey i have a 2003 mustang gt and there is a problem with the heat that ive never seen before. when im driving my car and i have the heat turned on it sometimes starts blowing cold air out and the temp gauge will start to drop a lil then go back up. when i stop at a stop light it will start blowing freezing cold air but when i start moving again the warm air starts blowing. can anybody help me out and tell me what the problem could be?