With the AC on the stationairy rpm is reased, wich explains the smoother idle.
Between the injection system and the thermostat you can find the idle control valve (round aluminium tube with electrical connector on the front). On the right side (seen from the front) there is a small screw. With this screw you can ajust the idle speed. It may be hard or impossible te turn, iron screws in aliminium is generally not a good idea. I drilled mine out and replaced it with a copper screw (much longer for easy adjustment).