If it used to be more accurate and you haven't changed to a different tyre size or style, I can think of a couple of things that might cause this. Either the speedometer needs to be cleaned and relubricated, or the drive cable may be binding and snapping forward. I could be mistaken, but I have the impression that the speedometer has a damper that might smooth out the wild needle waving that this would cause on other speedometers.
