The IAC (Idle Air Control) controls the idle speed when load inducing accessories are turned on like A/C and Alternator loads. It increases the idle speed to compensate for the extra load. It does this by putting extra air in the engine with the IAC. It may need to be cleaned or replaced. It is a small electric motor connected to a flange on the top of the Throttle Body. It has 2 8mm head bolts and has a couple of wires going into it.
Take it off and spray the inside of the IAC cavities with choke cleaner spray. Let it soak for a while and then spray it again. Lubricate the pintle between the cavities with a light engine oil. Put it back together and you're done. If this doesn't help you will have to replace it.