Had to service a speedo last year and there is one thing you can do that seems to resolve most bouncing needles. The mechanism for needle operation is magnetic attraction between a drum and a spinning magnetic rotor. Lack of lube or a gummy mechanism puts a load on the pivots that can't be overcome at certain revolutions. Lightly spray the back of the unit with electro clean to remove dust and grime. Use the nozzle tube that comes with the cleaner. Spray sparingly to keep cleaner off plastics and painted surfaces. Lighter fluid is less reactive and also works very well. Lightly blow dry and the apply WD40 lightly on all pivot points using a sharp tooth pick dipped in WD40. Voila. To test your results chuck up an allen key in your cordless drill and spin the cable boss on the back of the speedometer from low to high speed and observe the needles movement. If it moves smoothly and holds with various drill speeds job's done. If not look to the speedo gear in the tranny or a cable that's dry or curved too tightly. Use only dry lubricants like graphite on speedo cables.