when I have had a problem with the speedometer jumping it has been because the cable is just about to fail. it has usually been because the inner cable is frayed a bit so it catches as it rotates. if you check your speedometer gear and it is ok, pull the cable and pull the inner cable completely out, check it for frayed areas and if there aren't any then coat the cable with lithium grease and put it back in. I have not seen jonk's technique but it sounds like a good one also. I usually squirt a little carb cleaner down it and blow it out with an air compressor before re-inserting it, but truthfully most of the time when this has happened I have not lubed it in time and I end up replacing the cable. I hope your problem is simpler. either lubrication or a bad speedo gear.