Comment 2 for bug 554637

You monitor does not provide an edid, so the xserver selects an mode based on the sync ranges and modelines you provided. If it does not select a mode you like, you can change it at run time using xrandr:
xrandr --output VGA-0 --mode <mode name>
Run xrandr by itself to see what modes are available.
Alternatively, you can force the preferred mode by adding a PreferredMode option to the monitor section of your config:
Option "PreferredMode" "1400x1050_85"
See this page for more info:
http://wiki.debian.org/XStrikeForce/HowToRandR12