I have 3-1250 Hp. Reliance drive motors on an overland conveyor. One of the motors seems to be oscillating when running, this is creating noise in the gear train due to the oscillations. I have strobed the H/S coupling at 1800 RPM and it appears the motor is trying to stop. When you look at the attached video you can hear the gearmesh noise when the coupling drops down. I have strobed the other 2 motors and you can see the same thing with the coupling and slightly hear some gearmesh noise, not as bad as the one in the video, but off course the motor in question is affecting the the other drive motors. These motors are running on a Toshiba VFD and they generally run at 1800 RPM when running at max speed. What could be causing this?