The idle speed control will cylce up and down if it is working properly (50 rpm + - should be acceptable). The 103 engines in the 88-91 time frame have what MB technicians call a "twitch" at idle. I replaced every component in the fuel system with excepion of the distributor and air flow meter. I still have the twitch. You can get rid of the hesitation by playing around with the mixture screw which is accessible through the top of the air cleaner. If you have it set by a mechanic using an exhaust gas analyzer it will continue to hesitate.

The A/C compressors in these cars have a difficult time keeping with the cooling load when everything is to specs. I suspect that you would be better off going back to R12. There is supposedly a compressor oil now available which is compatible with both refrigerants.