Always, always first try the simple things - there's always time later
for the complicated and expensive.
How do you KNOW the transmission is slipping?
These boxes start off in 2nd gear by default and that makes the car a
slug - so you may think it's slipping.
Try this: At a DEAD stop shift into L. Start off - that's 1st gear.
Upshift to S and IMMEDIATELY BACK to L to hold 2nd gear.
Then shift to S for 3rd and D for 4th.
Do you still believe it's slipping??
The smoothness or harshness of the shifts is controlled by a vacuum
valve on the throttle and a modulator on the transmission.
The more the throttle is opened the LESS vacuum to the transmission and
firmer the shift - conversely the less the throttle is opened the MORE
vacuum to the transmission and the smoother the shift will be. That
said, don't expect this old hydraulic box to shift like an
electronically controlled box on a 2006 model - it won't, for it never did.
Since you're new to this car you should know its maintenance schedule.
Engine oil & filter change at 5K miles, valve adjustment at 15K miles
and air, fuel filters and transmission fluid and filter at 30K miles.
Repair parts are widely available from second sources like
www.thebenzbin.com, www.performanceproducts.com, and www.autohausaz.com,
I've owned a diesel since 1980 and don't understand the fascination of
biodiesel but here's a site: http://www.biodiesel.org
The more care you give this old car the more service it will return to you.
Good luck with it!