SPeedometer

I know there's been threads on speedo sensor units and such- thanks. I have a curious problem- my speedo frequently runs itself to 70 and stays there, and occasional falls back down to zero at stops but mostly just hangs. I used to be able to gently thump the lens and it would function properly (no hate mail please- you've all done it!) but no more.

The curious thing is that my speed sensitive stereo continues to work perfectly adjusting teh volume up/down with road speed. Is there any connection between the stereo and speed senor or is it reading cockpit noise levels or ??

Curious.