My friends dad had a 600SEL in 1992 and had so many problems between 1992 and 1994 for steering vibrations, he invoked lemon law and got a new one.
He had a vibration, that would come on at 60 mph like clockwork.
Wheels and tires blanced, replaced and all, suspension replaced, steering box and all replaced, mounts, the works, essenstially they replaced the entire suspension, wheels, tires, mounts for the motor and tranny and all steering components and they could not get rid of it.
He had to take the car in many times for this same problem, under warranty, and by 1994 he was so fed up, he walked in and demanded lemon law be invoked and drove out in a brand new 1994 model. 2 years later he switched to BMW 750IL. he drove those for 6 years, then got the new s600 when it came out, the new bodystyle, and he loves it more than the bmws he had.
Now that car was a 1992 and had those problems, first year it was out.
So I doubt it has anything to do with your car, but I would basically take it back, and refuse to leave witht he car in your possesion until the problem is fixed.
If you have to take it back enough times for the same problem, you may be able to invoke lemon law and get a new car.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.