I don't know about soft springs making for fishtailing but when test-driving other 190s before my purchase I found one with bad links in the rear and boy was it sketchy on the highway. At 80k miles my 190 is starting to "break-in" back there and I'll notice a little wallow/gyration thing occurring at times (especially over expansion strips where you get an up/down undulation).
Related question: it seems as if the car/suspension is beginning to "crash" over certain types of obstacles (speed bumps, potholes, other sharp-edged nasties). By that I mean they feel harsher than I remember - I think (it's been a gradual thing). Could this be the shocks going away and an indication of bushings getting worn?