My 90 240 has a shimmy as follows:

1. When braking but only from 40 MPH down to 20. It seems worse when the brakes have been used some. It's repeatable eg. on a hill - coasting or in gear - it will do it over and over.

2. There is also a slight shimmy when turning at low speeds whether braking or not.

I've checked/ replaced almost everything in the front end: all bushings, ball joints, inner & outer tie rod ends, strut mounts, wheel bearings. Tires have been rotated multiple times. It was just aligned. I even swapped the driveshaft with another car.

I also replaced the rotors. It seems better. There is slight visible evidence of a hard spot on one rotor - they were on another car for ~ 3K miles and then they sat around. But I temporarily replaced that one and no difference.

I suspect that I may have some combo of: poorly balanced (Goodyear Integra) tires, a bad spot on a wheel bearing/ race, and a bad spot on one or both rotors. It's still hard to see why the shimmy would only be in the 20-40 MPH interval.
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240s: 2 drivers and some parts cars