Latecomer to this thread. I'm wondering if, by any chance, the OP might be using an old VGA cable to connect the external monitor. Modern monitors send Extended Display Identification Data (EDID) to the host computer, but some cables (i.e. older ones), do not pass the signals through on all the pins. When X doesn't get the EDID, it defaults to conservative settings (i.e. 1024x768) and convincing it to run at a higher resolution can be problematic. Not saying that it's a cabling problem in this particular case, but it conceivably could be.
There really should be no need to calculate mode lines and the like. I remember this kind of thing being done back in the 1990s, but today it's more-or-less a relic of the past.
I've found that once I start experimenting with xorg.conf (and /etc/X11/xorg.conf.d), these experiments can come back to haunt me later, especially if I forgot to save the original contents of these critical system files. I mean the contents that were put there by the automated install process or by the desktop environment's configuration mechanisms, which is what I try to use instead of manual editing, if at all possible.
In cases like this, I consider the option of simply moving my data to an external disk and reinstalling. I know the OP said s/he wants to avoid that. Still, what better time to do it with the external monitor not working and 12.3 now out?
Reinstalling is a realistic option for me, because I try my darnedest to make sure all my personal data (including config files, scripts, etc. - everything) resides in my home directory. If I have some home-grown scripts or config files that need to be in /usr or /etc, I put them in /home and symlink from there out to the filesystem. That way, when I need to reinstall, I can just offload my home directory and bring everything back after the reinstall. There's a little bit of work reinstalling applications, etc. and configuring them so everything works the way it did before, but on the other hand I have the benefit of a fresh install, with no skeletons waiting to fall out of the closet in the form of intractable bugs in the setup.
Hope this helps someone.
Wednesday, March 13, 2013
xorg.conf, modelines, EDID
Here's a forum contribution I wrote concerning xorg.conf, modelines, EDID, etc.