I have a 87 300D, 124.133.
Your problem sounds similar to a trans shifting problem I had last year. It progressively got worse over about 2 months. Started out flaring on the shift and then would just seem as if it went to neutral on the shift. I was able to manually shift it for about a month, but then it got to where it just would not shift. (Manually shifting it raises the line pressure)

I poked around on the internet and found a Russian transmission site where transmission mechanics posted problems to other trans mechanics. Don't know why they used english, but they did.

One of the guys said to remove the rear valve body and check the mesh screen that was contained within one of the oil channels.

Just as he described, this fairly course mesh screen was plugged with clutch particles (not clutch dust, but chunks). I cleaned out the particles and the trans has worked since. Got about 22K more miles on it and drive it every day.

This is the rear most valve body, not the large one.