This is such a coincidence. I have a 1997
stang and they same thing happened. I just got it fixed at the dealer since i knew some guys there. They charged me 185 bucks to fix it. I replaced the VSS, speedo head, vss plug and the driven gear. the gear in the tailshaft was ok. I had some work done to the computer earlier. When ever that happened somehow the ground wire to the speedo was disconneted. My suggestion is get in that kick panel and start chasing wires with a schematic or something that will tell you what is what. Also if you have a performace chip in there take it out and see if that fixes it. if it does then get your chip re-burned. I hope for the best for you and your car. CHECK THOSE WIRES!
Steve