How is the cooling?
And also the age of the transmission oil and filter?
Here's a little history of similar events on mine.

First.
Very old fluid was making the box rather clunky. At the time, I didn't have access to a filter, so I only drained the box, and refilled with new fluid. It made a world of difference.

Overheating cooling system.
When I had an issue with overheating, a side effect was that the transmission would frequently lock in one gear, and stay there, until the ignition was turned off.
Initially, it didn't appear that I was having a cooling problem.it became gradually worse. My supposition was that gearbox cooling circuit was suffering simply because the engine cooling was not normal.

This also manifested mostly in stop start/ slow speed traffic.

My cooling problem was actually a failed viscous coupling for the fan. Worth checking. Have a look at one of my threads.

85 WR quattro
90 V8 Silver, 91 V8 Black, 93 V8 Lago
93 100e quattro V6. Titan, i think