Evaluating fluid level in MB trannies is way more difficult than it should be. Be sure you use the lower mark when looking at both sides of the stick (if there is a lower mark).
Being over full can cause your problem, but not right away. The fluid will have to foam first this usually takes some miles. I only see it on cars coming off the interstate. As a result, you should try the car again. If it still doesn't shift it isn't from being overfull.
The governor is a good possibility. A stuck control pressure piston would be another.
Bosch Master, ASE Master, L1
33 years MB technician