if the cable is lubed with no binding or kinks in the housing then its the speedo itself thats causing the problem, usually the shaft that the speedo numbers are atached to, the shaft gets dry, then it gets hard to turn, causing the speedo cable to wind up a little, when the cable gets wound up enough it releases the tension to the speedo shaft causing it to bounce, show a faster speed, then all the sudden tension is released the speedo slows down, then the process is repeated over again thus causing the speedo to show fast then slow speeds, clean and lube it, either at home or send it out, if that condition goes on to long it will destroy the speedo