The only way it will do this is the flywheel is not tight on the taper. Remove the key and use fine grinding paste to lap the flywheel onto the shaft. When you have a nice matt surface on both surfaces clean the paste off and assemble with a new key. Make sure that the washer is in good condition, not cracked, and there is no sign of the flywheel denting under it and torque to the manufacturers recommended pressure.
John Marshall, September 2009