Doesn't xubuntu use lightdm by default?
Regardless what you need to do is build the /etc/X11/xorg.conf, by running the following command:
sudo aticonfig --initial
This will make a generic xorg.conf but should do the job. If it still occurs then you may need to do additional (or follow the research you have done) in order to modify that file to your optimal settings.
One extra hint, after logging in use the command 'xrandr' which should display that monitor as "connected" and give a list of supported modes. perhaps use one of those?
If you want to go the DM init script route then according to Run a startup script with lightdm
"You might want to try adding pre-start or post-start scripts to
(see http://upstart.ubuntu.com/getting-started.html for an introduction to upstart that parses this file)
Also /etc/lightdm.conf allows to specify scripts to be loaded. You have a detailed description of all (many) the options that can be used in the configuration file lightdm.conf at /usr/share/doc/lightdm/lightdm.conf."
For example, for gdm, I resolved an issue by adding the following command to /etc/gdm/init/Default:
if xrandr -q | grep -q "CRT1 disconnected"; then
xrandr --output LVDS --mode 1600x900 --pos 0x0 --primary
So perhaps you can research into that answer (and the upstart link) to determine if you can add a script which basically does the same thing (tho you won't need the if statement, if only running 1 monitor)...
I had do this modification because after futzing around with xorg.conf to act like I wanted when booting from single monitor (laptop) to multi-monitor, when in single monitor mode, the GDM login screen was using the wrong resolution (it was using the multi-monitor virtual display size... I think), so issuing that BEFORE the initctl command for GDM resolved, and it should resolve for you too (I say should because I haven't looked into lightDM to0 heavily).