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Ths IAC should be open when you're idleing allowing air to bypass the closed throttle plate. If it was bad I would think it would also affect the idle when the ac wasn't being used. Something has to happen to tell the ECU to inject more gas when the ac goes on to prevent the compressor from draging down the engine speed. There's a relay somewhere that's involved with both activating the compressor clutch and also sending the ECU a wakeup call. There may also be some switch on the heater controls but if the compressor is going on I would think the switch is ok. There's a relay on the heat/ac unit behind the glove box but I don't know if that's the one. Maybe someone else has the location? Could also just be a compressor thats seezing up but you should get some idea of this by observing the belt when the clutch ingages. If the belt looks like its really straining and loosing it's grip the compressor may be hard to spin.