The vacuum modulator is what determines how hard the car shifts. The less vacuum the bigger load on the engine and the harder it shifts, which is why a vacuum leak would cause it to always shift hard, I just dont know if the modulator itself can go bad and cause hard shifts all the time.
The info I'm looking for is probably somewhere on these forums, I just gotta find it

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1992 190E 2.3
150,000 Miles