The AC is turned on in the defrost mode. The system computer adjusts the idle control motor to allow more air flow to keep a constant idle under the extra load of the AC. When the defrost is turned off, the AC also turns off and the idle control motor is supposed to close off some air flow to keep the idle speed constant. It sounds like the idle control motor is briefly over-correcting and allowing the idle to drop too low. I'd first clean the idle control motor, make sure it has a clean electrical connection, and also check to be sure the throttle switch closes in the idle position. Many idle problems have been caused by deposits in the throttle body. You might also clean this.