I've had several cars do that. Usually lubing the speedo cable does the trick, at least or a while. I tried the lube up idea on one car. I pulled the cable off the tranz and wrapped the tube of an aerosol lube to it with tape. After several tries and lube all over my face it worked. Then it bounced a few weeks later and I lubed it again, then I pulled the cable and lubed it from the top. But I think too much damage had been done by then and it was in permanant bounce mode.
My last car was a '91 Acura and it would not just bounce, but drop to zero, hover around 20-60mph higher than I was going, or even hover around 20-40 at a complete stop. Weird, must've been computer controlled. It rarely did it, not enough to bother fixing.