On those machines when it does it while the hopper is running it's probably because of the magnetic field created by a motor when it is operating. That magnetic field can affect the crt display and cause shimmy or other movement. Doesn't affect an LCD display because it doesn't use magnetics to move the electron beam like a crt does.
But for what your display is doing I don't know what would cause that. Something easy to try would be unplugging all of the fluorescent light ballasts, the ballast is a coil similar to a transformer and it gives off a magnetic field.