[Graham]

I think basically the grub screw in the crown gear was worn, and although it seemed tight enough, it did feel like it was slipping when I tried to tighten it further. The new one I put in seems rock solid, I hope.
Another axle stopper on the other side would have helped keep the axle centrallised,but if the gear was slipping on the axle because of the worn out grub screw, then it would not have helped the cars performance.