It also sounds as though you have the virtual desktop feature enabled. If you can "push" the mouse against the bottom or right side of the screen and get the screen to move around, that's what is going on. You should be able to find a setting to disable this from within X, but exactly how depends on which desktop environment you're using.
You can also manualy edit out the virtual reference in the Screens section of /etc/X11/XF86Config:
Code:
Section "Screen"
Subsection "Display"
Depth 16
Modes "1024x768" "800x600" "640x480"
ViewPort 0 0
Virtual 1600 1280 <<<< remove this for each depth
EndSubsection
If you're using XFree86 v4.x.x, the config file is /etc/X11/XF86Config-4, and the format of file is slightly different. I'm not at my Linux box right now, so I can't verify if it still uses the "Virtual" line.