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.
I haven't looked at the mustang dash cluster for a while, but usually when the speedo or tach bounces, its because it is dirty. If you open it up, you shuold be able to lube up the speedo mechanism.
You need a new speedo cable - mine did this at certain speeds - stopped after replacing the cable - its not to difficult - some folks claim they can reach up behind the dash to disconnect the old one and plug the new one in - I tried it but turned out quicker just to pull the dash to get the cable - its difficult to squeeze it to get it to release.