Your mechanic IS missing something. Having driven a 88 528e for 5
years, and reading lots, and dong my own work. I encounterd the
Worn idler arm bushing. This will effect toe in/out as the rh front
wheel can move back and forth in relation to the left. Steering was a
bit vague. Replace idler arm.
Rear subframe bushings fell apart. Car did not track straight;
constantly all over the road. Especially around turns where the rear
end will have a mind all it's own.
Front upper control (thrust) arm bushings fell apart. Not real
noticable except under braking, you could feel a clunk and the car nose
would dive to one side or the other. If your rotors are slightly
warped, this will exacerbate the problem and greater shimmy will be
Worn ball joints, so I replaced the entire lower control arms with new
bushings (original bushings were not bad though).
I did not have issues with the rear "dogbones", but others have.
Lots of archived info on http://bimmer.roadfly.org/bmw/forums/e28 /
about the e28 suspension.
This all took place between 120-150k miles.