It sounds like the dealer is on the right path of diagnosis, it's got to be either one of your sensors (FMU, MAF, ACM). They should be able to plug-in their DRB scan tool and see the error codes. But they probably don't know how to read them properly. I would suggest they have the DC regional rep. help them fix the problem if they can't figure it out.

I'll bet you a MB dealer could fix it in a heart beat...