Ill run my AC and it runs nice and cool(live in az its gets up to 120 here) but when i start down the road it tends to stop comming out the vents and start comming out the defrost vents. It seems if i let off the gas and let it slow down a bit they go back to normal, somtimes not at all. So at a constant speed ill be sweating my ass off. Any sugguestions what might be causeing this?