I have been been watching U-Tube videos several days with charging problems of my own.. There is lots on motors like Yours. One happened to look at the wires that came off of the Coil to the Plug and the wire had a break inside it. It would read sometimes and not at others .. The wire would Arc inside the wire and fire them misfire. Other than this the CDI unit that fires the coil Packs may be breaking down when the motor is warm --- Hope it is the wire