Your problem is most likely tires, but without driving it myself, it's difficult to know.
I have had a number of problems over the years getting the tire people to balance the tires properly. First of all, the balancing machine MUST be set up to add weight to the inside AND outside of the wheel if needed. It is very common for them to put weights on the inside only. Their response is always "most people don't want those ugly weights on the outside of their alloy wheels". Insist on balancing inside AND outside. A good alternative is using tape weights on the inside diameter of the rim as close to the outside of the wheel as possible. This is the ideal way because of the third problem, alot of the weights the tire stores use won't stay on a Benz wheel. This leads to what very likely is wrong with your car, one or more weights may have come off.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in