Geoff I was experiencing similair problems with my daughters 90, I had adjusted wheel bearings/adjusted swivels/ replaced track rod ends/ replaced damper/ changed tyres and checked tracking all to no avail. So I decided to replace all the bushes on the radius arms and panhard rod, it was whilst I was about to undo the axle end of the panhard rod that I found the culprit, the bolt had been worn through on the inner side of the bracket and just needed a new bolt fitting and that is it now cured. Not saying this is your problem but it could be a quick and cheap fix if it is.