You probably have to enter some information in /etc/X11/xorg.conf to have X skip the automatic configuration.
Do you know that it's just the monitor that is being misconfigured?
In that case I think you could add the vertical and horizontal sync ranges to the monitor section.
So open up /etc/X11/xorg.conf in your editor (sorry, I don't use xubuntu, so I dont know which editor it uses, but remember to put 'sudo' in front of the command).
The monitor section should look something like this:
Note that you have to know the specific sync ranges for your specific monitor, so you will have to find the specifications for it before you do anything else.
Identifier "Configured Monitor"
If the problem is with your graphics card and not really just the monitor, you will need to give more information about your computer, i.e what graphics card you have and which driver you use.