what causes speedometer to jump
Dirty gunky grease on your cable is the cause. At lower speeds the cable binds slightly and then jumps when the binding is overcome. This causes the spedo needle to jump. I believe that the cable comes out from the spedo end. Pull out, clean off the old grease, use new graphite type spedo cable grease on the cable and reinsert. You may have to take the end loose at the transmission to reinsert the cable into the drive. Good luck,