Wouldn't doubt you have tire or head bearing issues but they would only magnify it. It happens because of the undulating pavement under the tires and the almost straight up fork angle of a sportbike and the 2mm change in for/aft pitch that you can't help when you let go. You'll buy brand new tires and punish your head bearings with torque and you'll only lessen it. There are three ways to eliminate it - one, ride only on perfectly smooth, level, straight surfaces; two, get a damper or three, put your hands on the bars. Hell, there are people who still think that it's the centrifugal force generated by the wheels rolling that keeps a bike upright.