Kind of weird that it is bad at 40, but goes away at 55.
I would check your tire balance anyway, and get them balanced at a shop that uses the Hunter GSP 9700 balance machine. The MB dealer and some independents use this machine. Access the website, www.gsp9700.com
and enter your zip code to find shops that use this machine. Trust me, it's worth it. This machine can figure out if your tires or unbalanced, or if they can't be balanced if you have a wheel force vibration problem. Your problem might thus be solved with new tires or a proper balance. This site also gives a great lesson on the sources of vibration. Read it. You'll learn a lot.
I'd get your suspension checked out, in particular tie rod ends, which can cause a vibration like you're describing.
Check your brake rotors for out-of-roundness. This can also cause vibration. Some people have criticized this idea, but it's clearly listed as a cause of vibration on the website for this Hunter wheel balance machine, and it eliminated a vibration on my Infiniti G20.
It may also be your driveshaft that's vibrating, so check the conditin of your flex plate.
Also, is your mechanic a Mercedes mechanic? If not, go somewhere else. Immediately.