The first thing I'd recommend is a fluid and filter change. If the filter is clogged a bit with use and age, I think that can lower the system's hydraulic pressure. And I have had excellent results with synthetic Dextron-III ATF - currently using Quaker State 4x4 fluid.
I had a problem that started showing up in cold 3-4 shifts. The shift would not execute and the car would just rev. New fluid and filter completely eliminated the problem.
(refers to my 1982 300D)
(also have a 1987 300D)