The 722.6 is electronic shift, you need to scan the transmission module for codes. Changing the fluid isn't going to fix anything.

A common failure point is the input and output sensors on the conductor plate. ( top of valve body, easy to change. )

Another common point is a leaking trans adapter plug / sleeve. You might not see a leak on the ground, but an o ring can fail, oil gets wicked up the wires to the trans controller. The controller can be cleaned with electronic cleaner.

Is there anything strange happening with the dash / fog lights ? I have a 97 E320 ( inline 6 ) that I'm using for parts. It is stuck in 2nd, high beam light on dash comes on,some warning message in the dash and the left fog light stays on. Once it returned to normal and shifted fine. I'm thinking a common ground has corroded and a few circuits are seeking each other for ground. ( MB puts a pile of grounds on a single bolt, this reduces ground loops but can cause all sorts of odd behavior when the stack becomes ungrounded. )