Voyager AirConditioning Issue

2000 Plymouth Voyager, 75K, 3.3L. I am not an auto expert but will describe the problem as best as I can.

Aircompressor and blower were replaced August of 2006 (16 months ago), and ever since the compressor has "pulled" the idle. This happens only when the compressor is engaged (air conditioning button is pushed), and not when the fan is simply on. The rpms go up accompanied by a dull roar, then rpms go down, then up with a roar, and so on. The heater seems to work fine. I am serviced by a auto repair shop with the best reputation in town, but they have not been able to solve the problem. The compressor burned out in June, and was replaced a second time. They have checked for leaks again and again and assure me there is none. They have replaced the idle air control valve as per a automaker bulletin. The problem persists. Thanks for any suggestions.