Have a Car Question? Ask a Mechanic Online.
It sounds like a vacuum supply problem to the AC controls; there should be a check valve in the vacuum line between the engine and the AC control assembly to prevent the symptoms you describe under low engine vacuum conditions, such as accelerating. If you find the vacuum supply hose, i would bet you either find a leaking vacuum hose or a bad check valve.
What happens, is when the vacuum supply drops too low, the vacuums ervo operated doors in the heater case move to their default positions, which is usually defrost vents and warm air on most vehicles.