Is it actually slipping, or is it just staying stuck in a lower gear? Your description sounds like the latter rather than the former.

Make sure you have the correct amount of fluid in there, check with trans at operating temperature and engine running. Low fluid level can cause all sorts of weird behavior.

As a last hail-Mary, the yellow bottle Trans-X can possibly help if you've got some varnish or goop clogging something up or making it drag when cold. Don't expect miracles, but like Nate said above, it made a difference on my car when it was "Shifted by Helen Keller".

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]