First thing to try is to inspect and lubricate the angle drive if you have one. If it's bad that could do it.
If that's not the problem, remove the speedometer. Put a large flatblade bit into a power drill and use that to spin the speedometer. At a constant drill speed the speedo should be rock steady. If not, have it serviced as there may be contaminants on the plate.
If all is good there as well then replace the cable. I don't know if the one from the local parts house will work or not, I've always ordered mine from Moss.
Good luck!