I am not an expert on M-B service, but my fiancee's C280 has had this issue. At first, she thought it was the battery. This may have been a factor, but the bottomline was she needed a new wiring harness. You see, this harness is like the central nevrous system of your electronically-controlled car. It is a very pricy repair, but you can push on dealer to fix at no cost to you under a "factory goodwill" program.

My gut tells me a lot of folks are dropping C class models on the used car market (she bought hers less than a year ago) because of the wiring harness and hassles with getting the free repair. Though this tarnishes M-B in my eyes to some degree, at least they are standing behind their design's apparent reliability weakness.