'87 SDL transmission not shifting properly

My '87 300SDL shifts very hard between 1st and 2nd. Since replacing the vacuum modulator, the rest of the gears are fine. However, a few days ago after the car sits for a few hours it does not like to shift out of 2nd. After driving at a maximum of ~45 MPH, if I come to a stop and start over it upshifts to 3rd and 4th fairly smooth. Is there an easy fix, or am I looking at rebuilding the transmission?

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Transmission / Suspension Specific Aid

Chassis Model Number

W126

Sounds like valve body problem

Try adding TransX additive to free up sticking valves. CLICK HERE to read more

First better, now worse

I added the TransX additive, and initially it made the transition much smoother. When I would start the car in the morning, it would shift hard from 2nd to 3rd the first time, but would be quite smooth as long as I was driving. Only after shutting the engine off and restarting later would it repeat the problem.

Recently, the problem got worse, as it is only shifting between 1st and 2nd gear. I was advised to rebuild the transmission or get rid of the car. Any advise?

Most likely internal

Might be just the valve body or could be something else in the pressure pump. If you opt for a rebuild buy a rebuilt unit from a reputable Mercedes transmission specialist. It is getting hard to find good used ones.