re. speedo problems. One guy told me years ago that if the cable breaks it will make the speedo work strangely because the broken ends of the cable still touch & interact, but ...... anyway, that's one theory.
A personal odometer experience on my BMW car: the little drive gear on the shaft had loosened after 35 years, it was originally a press fit onto the shaft. So I removed the speedo/odometer unit and was able to use the proverbial Large Hammer to once again "press fit" the gear onto the shaft. Has worked just fine for the last 5 years, so I reckon I've got 30 more years to go before it again needs The Hammer.
Cheers,
Carl