I have had similar problems with my tranny as well. As one of the moderators stated "time and mileage are catching up with your transmission".
I pulled the tranny out to change the front and rear seals, tranny pan gasket and filter. I also changed the tranny throttle cable (as I broke the clip removing it). I also renewed the flex plates and center bearing supports on the drive shaft to take up the slop. These tasks arent too difficult, but do require some ambition. And stamina.
These measures gave my tranny some more life. While it doesnt feel 'new' its still chuggin' along.
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)