hello, connect to the videocard's VGA port and try adjusting the refresh rates
Using Windows 7:
Right-click on the desktop, click
Screen Resolution
Click
Advanced Settings
Click
Monitor 
Put a check mark on
Hide modes that this monitor cannot display
Select the lowest support refresh rate. Click Appy. Restart your computer. See if the problem will persist after restarting the computer. If everything works fine, adjust the refresh rate again this time to the highest supported value. I hope this helps
Also, make sure that you are using the latest drivers for your videocard.
NVIDIA driver download page is here
http://www.nvidia.com/Download/index.aspx?lang=en-us
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You can also try the automated detection feature
http://www.nvidia.com/Download/Scan.aspx?lang=en-us
a JAVA plugin will be installed on your browser. It will detect the model of NVIDIA Graphic card that you have, and it will suggest the correct driver to install.