Hi,
When the cable housing(s) get dry and worn, this happens. If you have a pre 84 or so with cruise, the old mechanical transducer can cause it too. Finally,the speedometer head lube will dry out with age,and the mechanism can do this too. I would start by inspecting the speedo cable. See if it is one continuous cable, or if it couples together below the master cylinder and brake power assist stuff. The cable on my 83 is two coupled together, and I have been unable to separate the two. If you have a single cable, you can remove the instrument cluster,and remove the cable trough that end. Clean it, lube it with a graphite compound,the reassemble it all to see what has improved. Otherwise, replace the cable,and see what happens. If you still have a shaky needle,suspect the speedometer itself.I will discuss this further if you need clarification. Please provide some on your end. Good luck with this irritating problem.