Your problem is that you chose several X11
display-sizes ... the largest will always be the
res of the desktop. If you have decided to enable
640x480, 800x600 and 1024x769 and for example
made 800x600 your default resolution you'll have
the "scrolling desktop" phenomenon.
Solution to the problem:
Find the Modes ...
Remove the too high resolution, put the one
you actually want first.
For example, change:
Modes "800x600" "1024x768" "640x480"
Modes "800x600" "640x480"