I have a Filzer dB4LW. Works great except max speed display. I have tried new positioning, battery, etc.
I noticed something that the other posts don't seem to hit on.
Mine freaks out only when I exceed about 49 miles per hour.
So, unless there is an oddly coincidental solar flare every time I hit that exact speed range, I'd say the programmer never thought anybody went over 50. When the unit gets that info, it gets into an overflow.