Could be worn out tie rod ends, but I'm inclined to think it's a tire balance problem or a bad tire.
Are you having the dealer balance the wheels?
Are they using a Hunter GPS9700 balance machine?
Are they using wheel weights that stick on in the under side of the wheels? If so, that's your problem.
Insist on wheel weights that are the clamp on type, and they should be placed on two planes - the inside and outside lips of the wheel. The wheel weights that MB wheels use are different than other types, and if the wheel weights are not the specific type for MB wheels, they will fly off.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".