Today a user's monitor was blurry and fragmented, very similar to a TV with poor reception. The monitor would also go black for a few seconds, and then the desktop area would move half off the screen, and the desktop would later flicker for a few seconds again, and maybe the desktop would be centred again.
I tried a different VGA cable, which made no difference. I also reset the settings on the monitor, using the monitor's controls, which made no difference, except re-centering it.
When I swapped the monitor with a spare, the spare displayed fine.
Device manager showed un unknown Video device, and an unknown VGA Video device.
The desktop is a Dell Optiplex 780, which can have 1 of 5 different graphics cards. Belarc Advisor does not identify any card.
When I brought the faulty monitor back to the office, it displayed fine when connected to another Dell desktop.
The only thing I can't rule out is some possible electronic interference at the original site, which is disrupting the new monitor, but not the old monitor, although the new monitor was working fine for about a month.