Help needed diagnosing w123 transmission

[BlaueReiter]

Hey guys,
Complete newbie Mercedes owner here...recently bought an '85 300td w/211,000 miles. Florida car, pretty well maintained (for the most part). I've owned old BMWs for years and years and lately I've gotten smitten with the Mercedes marque. Have a new son and I'm drawn to combination of technological sturdiness, personality and safety. They're beautiful cars, really, and I'm ready for some tinkering. I'm a decent DIYer, as a matter of survival. But I might have stepped into the proverbial crap here.

Here's my situation as way of introduction:
My wife drove the car about 20 miles the other night, and when she got to our block, the car started acting like it had no power--engine ran, but car crept. It got worse, but she was able to park it in front of our house, but not without a lot of effort. She had to really floor it to get forward or reverse. She described it as like having the emergency brake engaged. I went out to the car and experienced the same thing.

here's the thing. I haven't been able to duplicate it. The next morning it shifted fine. I have some slipping/flaring going into 3rd gear, which could be clutch wear, but never any issue upshifting. Rough when its cold, but that seems normal.

I've been poring over the transmission threads to get a clue what might be going on with my new vehicle. I think I might have stepped into it by asking my mechanic to change the tranny fluid--it was dark and dirty. I should have read up some on this, as opinion suggests that if the fluid is dark and dirty, the POs might not have changed it regularly enough, and the fluid provided the extra friction the tranny needed. Now that the new fluid has cleaned the tranny some, I wonder if this could be the source of the flaring (which wasn't so pronounced before the change) and the incident the other night.

Fluid level looks right. Fluid doesn't smell burnt -- at least I don't think so (I'm rather new to autos).

Three mechanics mentioned the clutch wear revealed by the new fluid and said I should just grit my teeth and swap the tranny--it will need one eventually. Of course I want to rule out all possibilities before I plunk out a couple grand or more for a new trans.

should I be thinking about the 3rd gear flaring and the upshifting incident in the same thought bubble?

B2 piston replacment? Is it possible to tighten bands?

any and all suggestions thrown my way are appreciated. Need some education.

TIM
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'85 MB 300td
'67 BMW 1600