Why does the speedometer on a 1996 VW Jetta only work sporadically?

Hi, I had the same problem some months ago, the speedometer needle would work most of the times when I just started the car, but after running some time it would fall to 0mph, (and suddenly would work again after passing some rough terrain, that is the standard here in Mexico). I solved the problem dismounting the dash board, opening the rear part, where the PCB is, (circuit board), and I found that some soldering joints were broken, so I took my soldering iron and "retouch" the soldering points just melting the solder, I do not recall how I find the points that were desoldered by the vibration of the car, but it was easy to guess which points were broken (visibly). It solved the problem with my speedometer. It could be the same problem that you have, but also it could be the sensor gear with a teeth broken.