I believe you are at the point to let the ECU get checked.
The ECU is a fully analog, not digital unit. Some functional blocks have to be trimmed individually in each unit, and many repair-centers can only replace defective parts, but not trim the unit afterwards. Repairing and trimming takes a lot of time. The service therefore is quite expensive. And you can not be absolutely sure the workshop really has the knowledge to check completely, repair and trim these very special 'computers'.
You live in Switzerland - could contact Volker at www.oldtimer.tips/en
You will find a lot of information there, and Volker really has the knowledge to check and repair the D-Jet-ECUs. He is a Mercedes-guy, too you can communicate in English with.
It will be possible for you to send the unit to Germany without touching the customs, a great problem Volker normally has when trying to help guys all over the world - there is so much custom's-trouble then that it is almost impossible to handle it.
Here you can get information about the ECU, function and repair (English): https://oldtimer.tips/en/d-jetronic/53-ecu#Aufbau
Would also like to read from you in the forum there. You will find me and Graham there and some D-Jet-guys from Switzerland, too.