My speedo cable was jerky aswell, in my case the problem was where the speedo secures to the dust cap on drivers side front wheel.  the square end of the cable fits into a square hole in the dust cap but had rounded the hole so their was intermitent slipping causing the needle to drop and then return.  not having a spare cap I just epoxyed the cable to the cap.  Been working great for well over a year.  If the cap needs to be removed just chip away the epoxy, worked for me. Jim

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