I would check the transmission fluid and make sure it isn't low on fluid. Needs to be checked with car at normal operating temperature, level ground, engine running, car in park with parking brake set. It may also be due for a fluid and filter change.

The MPGs you mentioned are excellent for a large heavy car, your 1981 300SD weighs as much as two 1990 Civic hatchbacks.

I am sure that more here that are more knowledgeable on these cars will chime in. Also, welcome to the site.