The opinion that one should not change the tranny fluid is a widely held one, I think perpetuated by tranny rebuild shops, I am a big believer in changing the fluid and filter every 12K miles.

It would not hurt to try changing the fluid/filter and perhaps the modulator valve, if so equipt. If that doesnt help. then

http://www.autoshoppingcenter.com/transmissions/mercedes4.html

here is a link that has a tranny for your model for $1600.