Have you tried to apt-get update/apt-get dist-upgrade yet? There might be fixes available. Barring that (since I doubt that it is a problem with stock Debian), what graphics card do you have installed? Do you have the most current drivers installed and are they installed correctly? What chipset does your motherboard use? Have you looked up possible conflicts between the two? Lastly, which version of xfree86 is installed? You might try upgrading that to the latest version. Just some thoughts, as I have not had the same problem you have (though I seem to have been bitten by the Via/Nvidia incompatibility problem >).