Alex: I've experieced all those same problems with 124's. The sliding rear end was caused by tires that did not have good wet road traction. Using tires with top shelf wet road traction seems especially important with a 124.
The hopping wheel is classic shock abosober that needs replacement. Stick with Bilstein, Sachs or Boge. Of course, the many pivot points and links in the 124 rear suspension need maintenance, and that is the third thing I would check after tires and shocks. Ned Kleinhenz '95 E300D x2 '85 300D '80 300TD
