Its possible that you have a bad sensor board in your motor. They do go bad from time to time and have seen some from the factory that were bad as well. If your using a Tekin speedo you can use the sensor check feature to see if they are all picking up or not. I cant remember which Mode it is but Randy Pike may be able to let you know on here.
Other than that Sensor cables are common issues if they have some age on them due to the wires being so small.
EA