First, I'm not a trained Mercedes Benz mechanic but if it were my car, I would run a compression check and/or leak down test to see if you have a problem with piston rings.

And......not to sound preachy, but training family members in basic car maintenance procedures will save you a lot of headaches. An ounce of prevention is worth a pound of cure. IMHO, women only want to turn the key and DRIVE, they don't want to have to learn anything about the upkeep and maintenance of a car. In a woman's world, the only reason to keep a man around is so he can work on her car.