Ive never dug into a Benz trans yet, (I will be soon) though my initial impression of it that I've gotten around here is that it does get spendy if you have to open one up. One other thought I had after posting last night, the slippage could also be caused by worn out clutch discs beginning to slip.
I've taken a course on transmissions and rebuilt several domestics, which is where I'm basing this diagnosis on. Even though auto trannies have several different setups and control systems, the basic mechanicals are all the same that actually propel the vehicle (servo pistons compressing different clutch packs in a certain order). 2 basic types are the Simpson and Ravenaux gearsets. Honda interestingly is unique, in that they actually use a modified manual shift trans with solenoid controlled shift forks. I have a trans in my 240 that is doing the same as you describe, only in 4th gear, when upshifting, accelerating normally or hard, or going up a hill it slips. I am picking up another car this coming weekend with a dead trans and plan to tear that one apart to see how the Benz units are built (since that ones already totally dead) and maybe between the two trannies I can make one working unit. We shall see, I'm always open for a learning experiment.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale