Could also be the cluster too because if you spin my speedo cable with a drill it reads fine but if you, drive down the road the speedo bounces anywhere from 20-120mph but if yours does not do it all the time then it should be your cable.
Yeah, when mine is cold it starts bouncing at 15-20 mph. And when it bounces then it makes a full sweep from 0-140mph somtimes. When it warms up a bit it starts at like 50, and in the summer it only does it after 75. The warmer it is the less range it bounces in as well. In summer it only bounces from like 75-90.