The speedo on my 71 cb 350 worked great for 50 miles or so, then it dropped down and started hovering around 10 mph. when I would stop it would drop to 0. As I accelarate it will climb up to 10 and flutter. Then for a while won't move from 0. Then all of a sudden I will look down and it is working perfectly. And then it stops again. I checked the cable and it seems fine. Why would it function intemittently?