I had the same problem with my 2000 s type and the garage told me it was wishbone bushes, and this was confirmed when I put it in for an MOT. Hope this helps.
I have a 56 plate s type diesel which most mornings goes into restricted performance mode. It seems to be worse when cold. Ive been told its a blocked DPF filter. Can this be removed by specialist succesfully? and if so does it work? Many thanks Dave.