There is good chance that the problem could very well be related to arcing electrical lines. The reason the problem does not affect the VGA monitors is because they operate at a refresh rate of 60 Hz. The 60 Hz noise occurs at the exact time intervals that these displays are in refresh and therefore does not interfere with the screen display. When higher refresh rates are used, as in the case of Macintosh Displays, 60 Hz noise can adversely affect the display.
There are two things to try. First, turn off all the fluorescent lights in the office. If that cures the problem, you most likely have an arcing fluorescent lamp operating near the monitors. Replacement of the defective lamps or ballast should cure the problem. Also, try turning the monitor on its axis to see if that helps. There are no practical shielding solutions for a problem of this type.
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