The only time I had my speedo going crazy was when my alternator wasn't working at all and all the electrical systems were running off the battery and draining it. At one point from jumping like crazy, the speedo needle just went to 0 and shortly after I lost all power.

So all I can suggest is checking your alternator and the wires from it, if it's charging properly etc. After that I would give your speed sensor on the trans a visual check. A friend of mine once had a similar issue, actually. He had a 420a Eclipse and had to change trans, so he swapped it with a trans from a PT Cruiser but the speed sensor was different so it was reading really high speeds because the ratios were off. But if you haven't swapped trans or sensor that shouldn't be your case.

We had this happen to my friends 97. It was bouncing like crazy... We recently did some engine bay work and forgot to re-install a ground wire. Once re-attached the gauge worked fine. If you have been working under the hood check your grounds.