Yet another tranny issue - any ideas?

[Tubularfab]

1987 300d that I'm driving and working a trade with a friend for. 252k miles, no service records. No idea if tranny has ever been touched...

Two different times Ive had a strange occurrence where it shifts fine for several miles of driving, then go some reason decides to act up. It will shift 1-2, then on the 2-3 it acts like it goes into neutral and engine revs freely. Slow down and it returns to 2nd - then does same thing if you accelerate. I am quite certain that I was nowhere near full throttle and thereby hitting the kickdown switch.

The "fix" both times was to stop, shift into reverse for a second, then go back to drive. Drives fine after that...

What does this soun like? Piston hanging up somewhere? Sound terminal?

Thank you