Try reducing the screen resolution from 32 bit to 16 bit and see if that makes any difference.

Also try different refresh rates. Perhaps your monitor is flickering because the refresh rate is too high... or too low.

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Right-click anywhere on your desktop.
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Select Properties.
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Select the Settings tab.
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Click the Advanced button.
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Click on the Monitor tab.
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Click on the Screen refresh rate drop-down list for the settings that you have available to you.
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Select a new refresh rate, if you like, and press OK. Your system will try the new rate, and give you 15 seconds to approve it. If your screen goes completely nuts, do nothing. After 15 seconds without a response from you, the old settings will be restored.

YOu say the LCD is fine, but have you tested it in another PC to see if indeed it is not the problem? Have you tried another monitor with your Dell to see if it might be an incompatibility?

It could be the cable between the computer and the monitor.. check the connections.

Also it could be a driver related issue.. make sure you get the latest video card driver for your video card.

How have you determined that your video card is fine? It could be overheating *works fine for a while and then acts up*. I'd double check it just to be sure.

Another thing to try would be to get a LINUX live cd (one that doesn't require you to install Linux to use it) and then insert the CD and boot the PC. Are you getting the flicker then with Linux? If so then you can rule out XP as the culprit.

YOu can download Knoppix LiveCD at the link below:
http://www.knopper.net/knoppix-mirrors/