  Here we study the instability of the vortex motion caused by vortex velocity
non-monotonous dependence of the friction force in the presence of a
gyro-force. We demonstrate that even a weak gyro-force renders the condition
for onset of instability more strict than for a vortex not subject to
gyro-forces: either the friction force must exceed a threshold, or its
non-monotonic velocity dependence must be a rather steep function.
