I have a 1987 300 D Turbo with 250K miles. At 192K, the tranny stopped shifting. Leading up to that, the tranny would have spells of revving abnormally high, before shifting to the top gear. When this happened, while driving, I would shift to neutral for a few seconds, then back into D. This seemed to assist the shift. I had the tranny rebuilt and they did not replace the "valve body". This is a breadbox sized box that looks like a maze, which controlls the shifting. With wear, the valves inside will stick and the engine will need to rev abnormally high before shifting. Once this was replaced. The tranny worked like new. Find an experienced tranny mechanic and ask about this. Valve bodies can be purchased as a re-build or new. I think that I spent about $400. I do not know if your problem is the same, but it may be.

Good Luck