An adaptive wind turbine generator (WTG) control system comprising a measurement system MS with sensors A1, A2, MC arranged to perform measurements of mechanical vibrations of one or more mechanical components of the WTG, at a plurality of operating conditions for the WTG. A storage unit SU stores, in a measurement database MD, measured data and corresponding operating condition OC data for the WTG. An operating condition manager OCM determines how to operate the WTG, including rotor speed PSC, according to a control algorithm in response to the data stored in the measurement database MD. Hereby, individual vibration measurements on critical components such as electric power generator and blades of the WTG can be taken into account in determining which rotor speed windows to avoid. This allow a more power efficient WTG, since a large safety margin around calculated or predetermined tabulated rotor speed windows can be avoided. Further, with a continuous update of the measurements, the control system can take into account variations in the vibration behavior of the components over time.