P11 rear suspension problems

HI guys, i;m getting this frustrating "wagging" problem in the back of my P11 (PRIMERA). It's the disc brake set up in the back. I changed the shock and put o orinigal P11, i even changed the "beam" or the banjo, the whole rear suspension in other words jus to solve the "wagginn" issue but it's still there. This only happens when i'm going at a high speed and i suddenly pull left and then right (as if dodging a pot hole or cutting in and out of lanes). i have 4 of the same profile tyres, i changed every thing in front also but i installed some NISMO shock and springs(front only) from the P10 model to stiffen up the front to handle better. do youa'll think it could be because i stiffened up the front and left the back stock ? or some other issue ?