722.6 five speed tranny has no dipstick. It also requires a very specific synthetic tranny fluid.
Take the car back to the original dealer, and ask them to pull codes. You'll probably need to contact the Zone Rep if the tranny repair was done more than one year ago if you want MB to cover the repair.
It sounds like the tranny is occasionally slipping into limp-home mode. When it does this, it will sometimes lock itself into top gear. Makes driving un-fun. Limp-home will lock into gear, and sometimes low or top. Sounds just like your problem...
I've seen this intermittent problem before, and on one occasion it was a faulty tranny temp sensor that caused the tranny to go into limp-home. It believed it was overheating. For some reason it wasn't storing codes.
The problem was that the dealership kept disconnecting the sensor, replacing the tranny, and then reinstalling the sensor. (This was the story, anyway)
In other cases, the 722.6 has required some new parts. In JCE's case, they had to rebuild the tranny. A friend of mine, who's car is under warranty, didn't get to find out what was wrong, as MB just swapped the tranny out.
My local service manager has dealt with a couple of these problems. I'll call and ask if he has any ideas...
1998 C230 "Black Betty"