I wonder if anyone can decode this madness...

Speedo behaves perfectly for 35 miles. I happen to glance down at 40mph and watch it smoothly move up to 125mph and stay there for about ten miles as my speed varies between stationary and 60mph. I stop, not for the first time, and it goes back to zero, appears to work but then sticks at 17mph for the rest of my ride. I get home and it has gone back to zero.