Hi all,
first sorry for english, but I don't feel confident yet in German.
I had the very same issues as you described and for me it was solved by changing the dampers. I put there Bilstein B4.
With that I changed also front springs and arms (due to worn bushings), but I'm 99.9% sure that the dampers solved the trouble.
I still I had issue with a knocking during rotation change of steering wheel, but that's another story...