Just took the bike out for first time and noticed the speedo bouncing allover the place could the wrong drive be on the bikes a 1965 thunderbolt.
Check condition of cable, even if new, disconnect from speedo head and observe cable end as you revolve the rear wheel by hand. It should spin true and not wobble from side to side too much. Check cable properly engages at bothe ends. Make sure cable not crushed and does not have very tight bends. Check condition of inner by withdrawing it. Often speedo problems are cable problems.
The actual drive from gearbox is quite robust but could be damaged. Only way is to check the teeth.
Could easily be the speedo.
Have checked cable seems OK but the speedo drive on wheel stop's and starts and makes a clicking sound when it stops winding then OK when working again.
Sounds like the worm is worn.
Trouble is it's brand new ah well looks like a new one again
You say the bike is 1965 - is it a 1966 season job, A50C frame, from autumn 1965 with rear wheel speedo drive, or is it a 1965 season job with gearbox driven speedo?
A50c frame in rear wheel