Just one final thing… (This is one of those maddening things that can send you insane) It’s easy to get caught out by the Monitor Preferences utility. If you’ve played about with resolutions in that, then it saves a hidden file ~/.config/monitors.xml with your chosen resolution.
This is parsed by something when you log in to the desktop and will override any settings you’ve made in a xorg.conf or on the kernel command line.
Aaarrrrgggg!!! Again, another undocumented feature!