Know how you feel, is it a 250 petrol by any chance? Took 4 dealer visits to sort mine, updated engine software first time(car 4 weeks old), fix lasted 3 weeks. Diagnosis next time said thermostat faulty so that was changed, this fix lasted 3 days. Another visit diagnosed previous fault so case raised with Mercedes when I mentioned rejecting car due to fault. Last visit was for new software from Mercedes, this took 2 hrs to instal, so far so good (4 weeks ago). The dealers at the last visit did say they had 2 other cases of the same problem on the same engine.