i would suspect rear sway bar llinks.

also does it change direction on accel/decel? if so the small horizontal link rubber bushes on the right rear may be bad and nobody noticed it. (i can't imagine this, but you have had it worked on twice and still have a problem though). this link (i dont remember the name for it) can be observed by looking near the front of the right rear tire. the rubber bushes are plainly visible and are known to fail at fairly low miles on the 124 cars.

though the rear suspension is similar, the 123 cars did not have this component in them.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.