Id start by chnaging the tranny fluid and filter. See if it improves any. Id also guess that time and mileage is catching up with your tranny. But on the other hand , there is a Benz taxi in my town , and that car has 500k miles and the driver swears that the original trannny is in there. (its a diesel)

good luck,

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Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)