The mechantronics unit is also called the 'valve body'...

A classic failure mode with automatic transmissions is failure to maintain fluid- fluid gets hot, begins to break down, BMW stupidly says "lifetime", wear particulates build up, wear increases...unitl a bit of something gets stuck in one of the small orificies in the valve body, or against a plunger, and the tranny is 'stuck'...a particular gear or function does not work- can happen in a bunch of locations, each with a separate symptom. Sometimes there is a bit of crap that floats around in a limited area- tranny works fine most of the time, but then when the part moves it can show a failure. (an additional failure mode is tranny and fluid gets hot, seals heat, shrink, degrade...hydraulic circuits cannot hold pressure, shifting pressures degrade, wear accelerates....this is much worse, as a rebuild is required. The symptoms of this are more 'continuous', and are worse when warm.)

If I was having issues like this, I'd try a few things. (And I KNOW this is absolute heresy on these BMW forums) and not necessarily in this order.

1. Complete and thorough flush of the transmission. Potentially reverse flow, with solvents.

2, Remove mechatronics, clean/flush.

I'd only consider this when the next step is replacing parts. Not much to lose.

GL

A