I've just bought a 5000 mile old 2013 F6b. I was out enjoying, my new stead and starting to use its power. On accelerating hard on a long sweeper the back end was definitely getting unsettled, giving a mild bucking under damped feeling. I was pushing 100 mph, it was less noticeable at lower speeds. Not unnerving but I felt it could do with stiffening up. Is there a preferred upgrade rear shock out there. Can you point me in the right direction?