[unclebubbles]

This is normally caused by a vacuum leak. The vacuum motors/air doors are operated by vacuum on most older vehicles, some newer ones have electronic actuators. There is usually a small vinyl vacuum line feeding the heater controls which goes thru the dash on the pass side near the evaporator housing . Theyre bad about dry rotting, crumbling from heat, or rats and mice chewing on them causing a leak. At idle the vacuum is strong enuff to overcome the leak, when you gas on it the vacuum drops and the heater controls/air doors move which sometimes causes hot air to be mixed with the cold and the air blowing from a different vent.