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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.

Any ideas?

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There are too many possible issues. Please answer the following questions:

First is the monitor a CRT or LCD flat panel? The CRT is way more sensitive to electronic interference. Put a magnet by a CRT and the beam moves around - the LCD is not affected.

Second, what was the screen resolution and refresh rate set to? If the screen resolution or refresh rate is not capable of being handled by the monitor, you get strange issues.

Third, you should identify the video card and load the proper driver. Do a search and search by the service tag. Look for the original system configuration page and it should tell you what the original equipment was. Failing that, open the system up and take a look. If you're in luck it will be a discrete card you can pull out and take a look at. If not, look on the motherboard for the video chip.

Fourth, could it be power related? Is it possible that the power was not filtered or was the monitor plugged into a socket that could have power issues (say an airconditioner or some other large load was attached so the power dropped?)

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