I'll second the endorsement on the factory manual. The bible is nice reading but not very helpful for repair. You will also need Steve Nervig's index, which makes finding your way around the manual much nicer (the factory CD interface leaves a lot to be desired).
http://web.mac.com/dakota/Mercedes/Home.html
Shift timing is controlled by a cable that runs from the tranny to the engine throttle linkage, and shift quality (hard or soft) is determined by vacuum. If your power brakes are working fine, then you probably have good vacuum output from the vacuum pump (more on that pump later), so as suggested you probably need to track down and fix vacuum leaks. On the transmission itself there is a vacuum modulator that can be adjusted, but you really need to first fix the leaks to make sure the proper vacuum signal is getting to the transmision.
Here is a nice link to describe your challenge:
http://www.continentalimports.com/ser_ic20242.html
Now, your vacuum pump. If it is original, you need to replace it. The original pumps have a crimped-on cover, the replacements have a screws holding the cover down. The original pump design is flawed and the pump can fail in as little as 125k miles (by 250k miles the odds of failure are very high); when failure occurs, parts get dropped into the front engine cover, which is also where the timing chain lives. Those metal parts will break the timing chain if they get caught and that will destroy the engine. You can find your vacuum pump by following the big plastic line from the power brake booster to the front of the engine.