Tacho’s out!

Hi together! I still have the problem that the speed indicator often falls to zero during the ride and thus also the odometer does not run any further. Did now have this checked by a mechanic who only unfortunately found nothing. He said he tested all cables and connections and sprayed in. The problem still exists, however, that it fails after a few KM, but then usually goes again. What can this still be about? Slowly it annoys me