Here are some tips on dealing with a mechanical speedometer and cable.
Lubricating the Smiths Speedometer Cable To Stop Needle Bounce I have no personal experience with TKO electric speed sensors, but I would suspect that they either fail entirely or work normally. If you're handy with electronics, you could likely bench test the cable and gauge by mimicking the output of a normal electric sensor without having to remove it. If I had to bet, I'd bet on a sticky gauge mechanism, or mechanical cable if you have one.