Changing the idle screw setting is the wrong approach. It's unlikely that the screw is changing position.
US model? Does it have an oxygen sensor? Does it have the Auxiliary Air Valve or does it have the throttle switch? AAV is most likely.
Have you set the base idle mixture? You need to be certain that there are no vacuum leaks before doing this so if you are confident that this is the case I can give you the procedure. It would be a good thing to do if you have had the car for a long time and it has not been set.
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'80 DL 2 door, '89 DL Wagon, '15 XC70 T6 AWD
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