Likely the Speedo cable is going bad. Look to make sure there are no sharp bends in the speedo cable. Larger sweeping curves is desired.
You can try to lube the cable. Lot of effort to removing the old cable, pulling the center cable, greasing and re-installing. Might be just as easy to replace with a new cable and know it will last many more years. $125 from Wilkinson Pantera.
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