[davids2toys]

When I bought my car the speedo stopped working or was not working correctly immediately. The cable is actually to short or has too much back and forth movement inside the sheath. Turned out to be two items.
It would be able to travel into the reversing adapter at the speedo to far, by this happening it backed out of the plastic gear in the tranny eventually killing the square drive opening. With Doug's(ERA) guidance, made a 3/8 long shim out 1/8 brakeline and installed it right on the cable at the adapter end. This prevented it from walking out of the tranny gear to far and ruining it.
Your problem of the intermittent jumping around sounds like it could be the end of the tranny gear. If I am right, it will get worse as the square drive gets more destroyed until just won't work at all.
The good thing is this gear was designed to be the sacrificial lamb, that is why it is plastic and cheap