Have you tried adjusting the shifting cable to the transmission and do you know how to adjust it ? another point is to check the modulating valve by using a pressure guage, otherwise, I'd advice you to send it to a garage who really know how to adjust it the right way, I have been doing this things for many years and I would honestly say its not easy. My credential: Chief mechanic and owner of an MB dealership. (just to let you know my comments and suggestions are base on reality and not just any guessing game) also doing it this way first may save you from having to spend or change your tranny prematurely.