Speedometer cable issue

Working on a speedometer head is much like working on clock. There are very small bronze bushing that shafts run in. When those bushings are worn enough, one rotating part kisses the other. I've seen the needle jump from zerro to 10mph before seeming to smooth out at certain speeds them begin really jumping.
There is another possible cause and cure... The cable shouldn't put any pressure on the instrument in any axis. The square port where the cable enters can be packed with old hardened grease and wear dust. Clean that square hole with parts cleaner and toothpicks so the cable end doesn't push on the spindle. Good luck with it.