Adrian, Alan,
Hydro:
Check ALL of the linkages for slop. the xx50 machines can develop A LOT of slop over the years in the linkage under the dash, especially if you have an early production machine with the "bent rod" linkage instead of the "rod end" linkage under the dash.
After that look for slop in the cam plate cam slot and pin.
Tighten up whatever you can find that's sloppy, then perform the "N" (neutral) adjustment as outlined in the service manual.......
....and rebuild the trunion as detailed in the FAQ while you are in there too.....