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