Why does the idle jump to 3000 rpm when ac is turned on?

I work at an automotive repair shop, we have a 2000 Ford Taurus 3.0L Duratec Vin "S".  Customer complaint was originally misdiagnosed and an IAC valve was replaced.  Customer returned and I found a vacuum leak at a broken PCV valve in the lower intake. We removed the intake and replaced the broken PCV valve.  After reassembling the engine, we test drove vehicle and found no problems and gave it back to the customer.  The customer brought it back complaining of an intermittent high throttle.  After several attempts to recreate the problem with little success, we returned the vehicle to the customer.  The customer returned several days later with the same complaint.  This day was hot and the AC was on. The engine was racing at 3000 plus RPM.  We turned off the ac and everything was normal.  suspecting maybe a defective IAC or even possibly the wrong IAC from a Vin "U" motor, we replaced the IAC again verifying the correct part. With the Scanner we can activate and command the IAC with no abnormal results to 60 percent with a slowly increasing idle. Once we command the IAC to 70 percent, the engine accelerates like a rocket ship to in excess of 3000 RPM.  We have verified part type, quality, operation, coolant levels, checking refrigerant levels now and even attempting to disconnect ac compressor from circuit to see if computer is solely responsible for idle jump.  Any Ideas?