My old 150 cdti has been handling "funny" since purchase a couple of years ago. Over time I replaced all the usual things: front and rear, A.r.b. bushes and drop links. Front: 2 new springs. Rear: banana arms and shocks, then new tyres all round. This all seemed to improve things but I sort of felt that one of the rear springs had "gone" although they looked O.K.
I changed both rear springs today and found on the passenger side the small bottom coil was broken and the drivers side spring was a heavier gauge.
Check the photo out!