Motorcycle Questions? Ask a Mechanic.
Your speedometer uses two mechanisms to operate the pointer and the odometer. The odometer is driven from a gear directly off the cable. The pointer is driven by a magnetic field generated by a spinning wheel and then passing through several other components in the pointer drive line. It sounds to me like there is a problem somewhere in the pointer drive line. It could be the magnet, pointer shaft, hair spring, pointer attachment, etc.
There are aftermarket replacements available but most all that I've seen are junk and don't work well or last.
Paramount Speedometer in California(###) ###-#### ***** ***** only place I've ever used to repair stock speedometers. It has been years since I've tried to contact them but I'd say they are still around. I'd suggest giving them a call and getting yours fixed. It is the most reliable way to fix your problem.
I hope this helps. Thanks, P.J.