For what it's worth, I'm with Button. The 'nodding dog's issue is indicative of something wrong with the front of the car and include cracked chassis, damaged stub axles as well as those suggested by Button. The most like causes are steering dampers not properly greased and free, loose tie-bars and out-of-balance wheels.
On the issue of "Nodding Dog", in My case it was the rear springs were too soft. I had removed a leaf to get a more compliant ride. Re-installed the leaf and no more "Nodding Dog".
