I had a similar problem on my 86' 300E. Initially I tried to change the filter and ATF. That carried me for about 4-6 months.
Unfortunately I eventually had to have my transmission rebuilt. If I were you, I'd try to change the filter and ATF first (cost approx $40 DIY). I paid $1,700 to have my transmission rebuilt using original MB parts (that included R&R). I was going to get one directly from MB for $1,500 but the deposit was high and they would not give you the deposit back until after they determine your core can be rebuilt.
I wrote about this on this forum. Do a search and read my previous post. It has all the specific details (deposit, transmission cost, MB dealer cost, etc.)