Oh yeah, sorry I just noticed that you said it's a month old. So it's a C230 SportCoupe. I haven't seen these act like this. Hard to believe it would just stall like that and leave no diagnostic codes for the tech to investigate.
It really shouldn't come down to asking for advice on the net like this, but happy to discuss it. I would tell them to put you in a loaner or rental car until they find what's wrong with it, maybe get a tech specialist from MB to look at it, they're some based right there in Chicago. It's an unsafe situation. Tell them that.