You need to find out what the specs are for your display, then edit the /etc/X11/XF86Confi-4 or /etc/X11/xorg.conf to make sure the horizontal and vertical refresh for the display are set correctly and the Mode Lines for the Video contain the resolution you want.. you can also try
dpkg-reconfigure xserver-xfree86
or
dpkg-reconfigure xserver-xorg
Xfree or X org will depend on Which branch of Debian you are running. Stable is using Xfree86 and Testing and Unstable are both using xorg
You may even find a X configuration that someone esles is using sucessfully here:
http://www.linux-on-laptops.com/toshiba.html