One other thing -- sounds like you're getting your tires balanced at a tire shop, not an MB dealer or MB independent mechanic? If so, they are probably using one-piece wheel weights. These have a habit of flying off the wheels. This has happened to me a number of times until I figured out I need to use official MB two-piece wheel weights. Dealers and some independents have 'em.
Might be worthwhile to look at your wheels and see if they all have visible weights -- if they're missing, it could explain why they do well for a while...