1990 740GL slow idle with A/C on

[brandjrb]

Hi all,
I am new to this forum and have just purchased a 1990 740 GL with the Regina/Rex FI system. It has 94,000 miles on it and has been maintained very well by the previous owner. What would cause an intermittent slowing of the idle speed when the A/C is on? Most of the time the idle will stay right at 750 RPM with the A/C on but at least three times a day when I come to a red light the car wants to stall, the idle drops to 500. It only happens with the A/C on. I checked the throttle body and it looks clean. There are no vacuum leaks. Am I correct that the ECU for the FI controls the idle speed? Could I have a flaky Idle air control valve?
Thanks for any input.
John