  We investigate theoretically multi-mode electromechanical "shuttle"
instabilities in DC voltage-biased nanoelectromechanical single-electron
tunneling (NEM-SET) devices. We show that initially irregular (quasi-periodic)
oscillations, that occur as a result of the simultaneous self-excitation of
several mechanical modes with incommensurable frequencies, self-organize into
periodic oscillations with a frequency corresponding to the eigenfrequency of
one of the unstable modes. This effect demonstrates that a local probe can
selectively excite global vibrations of extended objects.
