Hi AC,
I've got a 1990 300E so our cars are probably similar. Can't help you with the shifting problem, but the leaking transmission fluid could be one or both of the two hoses leading to the radiator. I had a leak in this general area but could determine exactly where. Replaced both hoses and now everything is fine. I think one of them was leaking at the fittings. The hoses are inexpensive and an easy job to replace.

Good luck with the shifting problem.

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Ken Fuller
1990 300E