Welcome to Benzforum. I'm not sure where you are located or if you purchased the car new. Is the car still under warranty, if so, is there a different dealer in your area that could service the car. It seems 18 trips to the dealer over a period of two years is extreme. Another avenue would be to search the internet for "Lemon Laws" for the State in which you reside. Keep us posted on your success.
Ken
Ken