Where are the best X tutorials, howtos, user forums?

I have a pesky and seemingly unlogged intermittent display
error with SL7 and Intel HD graphics controller.  X has worked
out-of-the-box for years, so I've forgotten how to debug it.
The arrangements of config files is different for Xorg 1.15.0
than what I vaguely remember (xorg.conf is not a file, Xorg.0.log
doesn't tell me what modeline is actually being used, etc.).

Without getting into the details (yet), I should learn how X works
in 2015 so I can ask intelligent questions in the right forums.

Google is becoming useless for literal search queries, so I'll
ask you folks where the best websites and howtos are to spend
perhaps eight hours reading, so I can get some sense of how
SL7 X should be configured, how to turn on debug logging,
how to force modelines (like a lower refresh rate), etc.  

After I've learned and experimented and taken notes and failed
some more, I'll come back with frustrating questions; or else
I'll report a solution or two to help others.

Thanks,
Keith

Software: it works because we say it works.  We don't need no
stinking testing.

-- 
Keith Lofstrom          [log in to unmask]