I had this exact problem on my 91 300D. The problem was the governor was hung up. This is an easy yet hard fix. The governor is behind a cover on the left rear of the trans. Remove the cover, remove the governor assembly, clean it, reinstall and problem fixed. (On my car anyway). The hard part of the job is getting the governor out of the trans after the cover is removed. There is a snap ring that is hard to get to with out a special tool. My local trans man didn't even have one. I got mine out just using whatever I could find.
Good luck.

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84 500 SEL (307,xxx miles)