Hi Dave,
I would still think the grub screw is your first port of call, just remove the shifter housing so you can look ito the top of the box, you will then see the u shaped fork we are all talking about, it is grub screw located to the shifter rod and if it slops around it will cause havoc. If you remove your reverse lamp switch you can reach the grub screw to undo it with a long allen key.
Pics???
JC

