Usually, when the needle is boucing, the input shaft-to-housing clearence is too loose causing the spinning magnet to hit the shell. Sometimes if you lube the shaft with a heavy oil (like 90w) or some spray grease (my 1st choice) you can get down the road for a while but eventually, you'll need to get the speedo shaft rebushed. The cable will usually work or is broken. If it's failing and hanging up, causing the needle bounce, the cable is frayed and needs to be replaced. Lubing it won't help much once it's frayed.
