How does it know your speed? I would not think there was a sensor in the speedo head that early monitoring the needle? (But my Dad borrowed a Riviera with an overspeed alarm on the speedometer that you could dial in) First thing I would check is cruise control vacuum servo with a mighty vac after you lube the speedo cable with graphite. you can get it powdered in a tube for locks. I dont think grease is the prefered call out for a speedo cable. Let us know what you find.
