First, check from your monitor's manual that the horizontal and vertical refresh ranges you have specified in "dpkg-reconfigure xserver-xfree86" are correct.
This page
http://reviews.zdnet.co.uk/hardware/...9148085,00.htm suggests that your HorizSync range is 31-61 kHz and VertRefresh range 56-75 Hz, but you should trust your monitor's manual instead.
After specifying the correct refresh ranges, it depends on the window manager that you use how it manages windows.