The air flow sensor position indicator (potentiometer) might have a worn spot at the idle position. Put a voltmeter between pins 1 and 2 (brown and blue/black wires). With key on, engine off, press down slowly on the air flow plate and make sure the voltage goes smoothly with no sudden jumps up or down. If you have an autoranging meter it is best to disable that feature to prevent confusion when it changes ranges. I had the same symptoms on the '89 260E.