Change the trans fluid and filter and dont forget to flush the torque converter. Benz automatics are sensitive to trans fluid changes and this has solved many a shifting problem. If you take it to a trans place they will tell you that it needs a rebuild and $$ later it has the same problem. Start with the basics.

__________________
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T