About the tires,
I had the same problem with my 86 300e. I had new tires put on, and the car ran fine. Then after about 1/2 hour of driving, the vibration started again. I took it back, and they re-balanced the tires. This time, they lasted about 2 days. Then, way out of wack again. The problem was that the clip on weights were flying off at speed! I'm not that crazy of a driver, I guess the Mercedes rims don't like those clip on weights. I told the guys at the Good Year place to get it right! Then they used these stick on ones that go on the inside. No more problems! The vibrations only happened at highway speeds. Around town, vibrations weren't noticeable.
Hope this helps

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Kyle De Priest
1986 300e
1972 BMW R75/5 (Best there is)