If it seems ot rock left to right it may be the rear sub frame bushings. There is a 3 point mount in the rear consisting of the two rear sub frame bushings and the differential bushing. Mine shifted around but after changing all the front and rear sub frame bushings it's nice and tight. The motor and suspension sit on a big sub frame that mounts to the cars chassis with 4 big rubber bushings. When they get old they colapse a little and get soft. After 20 years or 100,000 miles they all should be replaced to restore the good ride. I agree with Strife that the feame is not flexing very much, if at all. Good luck. MHO

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si