Did it do this immediately after the new tires and wheel balance? If it ran up to speed once okay, and then later started giving problems, I would expect you lost a wheel weight. Many of the tire stores don't have weights that will stay on the outside edge of your alloy wheels.

Another possibility is that they did not put weights on the outside of the wheels. I've had many problems over the years trying to convince a $5 an hour tire changer that I want the weights put on both inside and outside. Most of their customers don't want the weights on the outside because they look bad. One alternative is a shop that will use tape weights on the inside diameter of the rim as far toward the outside of the wheel as possible.

I would put my money on wheel balance being your problem.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles