2000 Honda CRV engine misfire when air conditioning is on

My Honda CRV runs great - never had any engine problems. Beginning today, when the engine is running and I am idling at a light, the engine idle surges, recedes, surges, recedes at about the same regular pace as a heartbeat. It ONLY does this when the A/C is on. As soon as the A/C is turned off, the car idles normally. It doesn't matter whether the car is in drive or in park-same problem. Any help you can provide would be greatly appreciated.