The air conditioning wowrks well until we accelerate up a hill. At that time, the fan continues to work properly, but the air comes out warm. In earlier years, the cold air would once again come out when the car was once again on flat road and under less engine load. Now, the hot air continues. If we stop the car and turn the ignition off and back on, the cold air returns until the next time the engine is put under a heavier load by going up a hill. Your suggestions are much appreciated!