Since the problem is happening when the AC is on, it sounds related to the idle air control valve (IAC). The IAC is responsible for maintaining the engine idle speed. When the AC is used, it should allow the engine RPM's to slightly increase to compensate for the added load from the AC on the engine. It sounds like it is over compensating. I would check that first to see if there seems to be a problem with it. Start the engine and unplug it. Next, turn the AC on and see if there is NO change in the RPM's.
Let me know what you find.
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Wednesday, November 13th, 2013 AT 8:44 AM