The idle motor or Air Intake Valve may have something to do with the idle dropping when the air is turned on. There is a method of removing, cleaning the AIV and reinstalling with some operations to allow the computer to reset to the new idle needs. Turning the AC on takes horsepower and so a new idle speed may be needed.
I can give you the basics.
Disconnect neg terminal
Disconnect AIV
Reconnect neg terminal
Start engine
Set RPM with stop screw
Turn engine off
Reconnect idle motor
Start engine w all accessories off
Run for two minutes
Shut off for two minutes
Run for two minutes all accessories on
The EEC1V computer should have "learned" the new idle requirement.
Thats how I set mine, and I used brake cleaner to clean the AIV. Once cleaned and dried I put some spray oil in the plunger. The car has run much better and my idle when warm is just a tad over 600rpm
