After about 20-30 minutes of running air conditioner the vents all shut and it sounds like the air is trying to blow out the defrost vent and the air gets terribly hot. NOTHING stops this except for turning the truck off for a while and letting it sit, then everything is fine and the same thing happens again after 20-30 minutes. Have had this at the Ford dealership and two other mechanics and no one can fix this. Have replaced just about everything..I am disabled and can NOT spend this money just throwing fixes at this truck. Anyone have any ideas? Have replaced blend doors and just about everything else.