I have a 1978 R80/7 that I have had since new.... I have noticed lately that on the highway at highway speeds the speedometer needle will start to gain speed. It will eventually go all the way up to 140 mph. The "odometer" is working normally even when the speedometer needle is smoothly hovering over towards 140 mph.

It is not erratic like the cable is binding. It is more like the speedometer is getting to much voltage. This only happens after you have been riding on the highway a while. It acts normal around town.

I was wondering if a part is failing; or, it I might have a bad ground??? Any ideas???