This may not be of any help, but it has happened to me so I will tell my story. I have noticed my speedo will drop to zero kph when I am riding on certain streets, then go back to the normal reading. I assume it is magnetic interference from either radio waves, hospitals or mobile phone towers. I have actually had my speedo reading 56kph/35mph while standing inside the house with the speedo in my hand, this is definitely interference of some kind.:confused:

This could be a factor in your erroneous readings, maybe.

CHEERS.

Mark