Mangusta1969,
You own a Mercedes Benz not a Chrysler. Take your roadster to a qualified MB repair shop or a MB dealership. I too, own a roadster with an automatic tranny. I am not specifically familiar with this transmission but I also own an E Class MB which produced similar symptons. My E Class has the same type of shifting lever, pattern, and functions as the Xfire. The problem was solved by replacing the shift control module which is located directly below the shift lever. The MB mechanic found the problem almost instantly. When in doubt I highly recommend going to a qualified MB repair shop. It will save you time and $$$$$.
Cheers