I'll take a guess the traction control relay on the Relay Control Module (RCM) is going bad. This is a common problem.
There are a number of DIY easy things to try, but with you not being able to work on the car and it being in a shop they seem out of the question.
I would hope the shop the car is in could use a scan tool that can get into the Mercedes specific codes as that may help get you started in the right direction. For your own info click on this link to get you headed to a possible solution. The post talks about resoldering some cracked joints, but in your case you want to replace the entire RCM with a new one and/or send the one that is in the car to tighed1 for repairs Myster No Start