Did you check if the monitor is connected to the ground?
Try connecting a wire between the shield of the monitor, the monitor case and the electronics of the monitor. Once done, connect everything to the ground and try to see if the fault has disappeared.
If the problem persists, try to connect the GND of the power supply with the ground connection of the monitor and check if it is ok.
Last but not least, try connecting a wire between the GND of the RGB connector and the metal frame of the monitor
These problems because there are sometimes ground loops that create interference.