I have an 85 500SEL as well, I have the same broblem and have had the suspension, allignment, and tire ballance inspected. I rotated the tires and the problem is still there. It has been sugested that the flex plates on the drive shaft may bee worn but as far as I can see they are in great shape. I have heard that you can get the tires ballanced on the car and that it is much better then having them balanced on their own. I have not found a place that will do it for me.