Pull your speedo gear out and see if the teeth are chewed up. It is on the driver side of your transmission, and held in my one tiny little bolt. It is on the end of the vehicle speed sensor (VSS).
If not that, it could be a galled up speedo cable. You have to remove the speedometer assembly to replace that.
I have replaced both, and mine still bounced. So I think the speedo is flawed/dirty.