96 Engine surges only with heater on

My 1999 Taurus engine idle speed increases about 100 rpm for a second then drops back to a normal idle for about 5 seconds. This happens only when the A/C or heater is on, not with the vent. If you don't have your foot firmly on the brake the car will budge ahead. It's fine travelling at speed.

Check the idle speed control motor. When the A/C is on, the rpm's should increase approximately 100 and stay there while the compressor is on. Also check to see how often the A/C clutch cycles. If it is the same as the engine surge, check for proper refrigerant level in A/C system

You got it way2old. Thanks