When an extension cable is used, the improper shielding of the video cable will cause a shadowing or ghosting effect. This improper shielding causes a mismatch of the input impedance between the video card, the video cable, and the monitor.

In addition, if using a switch box or low quality cable, these two items will also cause signal degradation.

Check the video cable to make sure all the pins are in good condition.

Test the monitor on a different computer.