I have the same problem. Sometimes the car is smooth, some times it ain't. If it is a balance problem it would be consistent. I suspect a worn bushing in the suspension, differential or trans. It the rubber is worn the component may shift into a position that causes vibration sometimes and then works its way back to smooth. Sounds simple but finding the real cause isn't.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T