I'm sorry to hear that you're having such problems. I guess I've been lucky with mine.
If you'll give the dealer a chance to work through it and make sure that you get the regional MBUSA service representative involved, I feel quite certain that you can get past this and have a reliable car for the fifteen years that you're looking for.
We bought these cars very early in the production cycle, which sometimes produces problems such as this. Once you get past the problem, you'll be past it.
Hang in there,