When the a/c comes on, the computer tells the idle control motor to bleed more air to the intake, thus maintaining the idle speed. I can't comment much further on your questions.
I'm wrestling with the exact same problem on a Mercury product in our family. I have a feeling the problem may lie with dirty injectors in my case. This may also be the case with your car. Your problem may be everpresent, and only manuifests itself during marginal conditions.
Keep checking everything, including the egr system.
95 E320 Cabriolet, 131K