I cured a bouncing speedo by shortening the cable a bit. On the transmission end, remove the cable from the tranny and then remove the gear, held on with a clip. With the end of a micrometer, measure the depth of the hole in the gear that the cable goes into and compare that measurement to the length of the cable end that goes into the gear. The cable is probably a bit longer than the hole is deep. If so, remove slightly more than the difference and reinstall. Check the other end as well where the cable goes into the reversing mechanism on the speedometer itself and do the same thing.
Suggest you join SCOF as there is a large Technical Section on the Forum available to members that addresses many little fixes such as you these.
