How firm the shifts are is controlled by the engine vacuum via some kind of vacuum modulator. There are a number of vacuum "switches" in the system as well.
I recently had a very similar problem on my '83 240D (which has a fairly new transmission), and it turned out to be a lack of vacuum to the transmission caused by a faulty vacuum switch .
By the way, I have first hand experience of 3 different Mercedes transmissions lasting between 180k and 205k miles before requiring replacement. The 205k one was my 240D above, so hopefully your's has a long way to go.
I would strongly advise you find yourself a knowledgeable independent [not the main dealer] Mercedes mechanic and get the car properly serviced on a regular basis. Over the long haul it could save you a lot of time and money.
Morgan Hill, California