a vibration at a speed over 55 mph

What is your question?

This car has 130,000 miles. When I am on the road going 65 mph or more, there is a vibration which develops and I have to keep raising or lowering my foot on the accelerator just to minimize the vibration. I cannot use the cruise control because the vibration is very bad. The tires are balanced and two MB dealers said they don't know. What is the matter with this car?