Ryszard,

As an experiment, get a long heavy duty extension cord and use it to plug your monitors into a wall receptacle that is completely isolated from the rest of any circuits from the circuit breaker box to your other equipment. A "dedicated" circuit. Know what breaker controls that circuit and make sure everything else in the circuit is off: Ceiling lights, table lamps, phones, anything etc. If it works then at least you'll know where to start.

If you have part of your monitor system plugged into one circuit and the others to a different circuit that cold be an issue as well. Worth a shot. Helped me a few times.

Dan