I have a 1995 Suburban and it takes forever to get the heat to get hot and then the front defoster never get beyond luke warm. The vents sticks when going from floor or front vents to defroster, but it does change (it takes a few minutes). There is plenty of air flow from the defroster...it just never gets hot. The vehicle has front and rear heat/air. The a/c had to be rebuilt last summer because it wouldn't hold freon. Do I have a heater core problem or is it a control unit issue?