722.3xx 4th gear comes and goes

[240dddd]

I'm sorting out a new-to-me 1987 300D with a 722.317 transmission. It has had shifting troubles of the 'not wanting to go into 4th gear' sort.

Here's what I've done so far...

1. unplugged, removed the kick-down switch (result: no change)
2. replaced and 'tuned' the modulator. I found the thrust pin to be broken. (result: shifts were softer but 4th gear problems remained)
3. adjusted the Bowden cable (result: no discernable change to the 4th gear problem)
4. drained tranny fluid (including torque convertor), replaced filter and refilled fluid with DexIII. Spent a lot of time making sure the fluid level was correct. (result: transmission shifted great! 4th gear was back! For about a day!)

The fluid I drained in step 4 was really dark and the filter looked very old (should have done that as step 1). Once I got the fluid level dialed in it started shifting beautifully. I took it for a long drive, both city and freeway, and all gears were working as expected.

Today I drove the car to work and it continued to work as expected. On the way home it was working great as well. I stopped for about 1.5 hours at my son's baseball training and when I started driving 4th gear was basically gone again. It seemed like it was on occasion trying to go into fourth but was slipping instead. When I got home I checked the fluid level and it was still right where it should be.

Can someone who understands transmissions better than I advise on why it would work so great for a day and then suddenly go back to losing 4th gear? Am I probably dealing with a dying tranny or might there be some other fixes to try?