It could very well be one of your wheels/tires. With my 560SEC the car was very smooth most of the time at highway speed, but would occasionally start to violently shake with the stock wheels and crappy old tires. My indy suggested that the PO may have used some sort of "fix a flat" product that threw the balance off. Once I replaced the rims and tires the problem went away.

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1982 280SL - Petrol Green
1994 E420 - Brilliant Silver
1995 E320 wagon - Smoke Silver