Well I went thru that a bunch of times, one broken speedo, one broken reverse angle drive, new cable etc my cure was a Smiths electronic speedo. Now perfectly steady, accurate, digital odometer
Assuming you want to keep you mechanical Smiths, things to try are lubricate the cable (grease not
oil) straighten any kinks in the routing - get underneath and make sure it makes only gentle bends - make sure connection at trans is free of bind, check the speedo gear for burrs or missing teeth (it is just plastic and easily damaged). Next is check your reverse angle gear, it should spin freely with no bind - it just unscrews from the back of the Smiths. Finally check your Smiths itself for bind internally - spin the inlet see if it moves freely. If any probs I suggest send it off to Nisongers for professional repair tho some take apart and service their own... All just IMHO
others can fill in their fixes
There is a ton of data on this over at SCOF as well.
