RH Linux 6.1, but with a kernel I built (removed a lot of
irrelevant-to-me stuff). Cyrix MediaGX chip. Possibly
relevant package list at end.

I leave X (and Gnome) running for long periods of time (days
to weeks) and this is usually fine. If it's relevant I can
probably come up with a list of all the things that were
running at the time: it was mostly a lot of gnome-terminals,
netscape and gnome's freecell when this happened:

X just hung. Attempting to switch "workspace" with window
maker (I thought it was the app I was using that had gone
bang) had no effect. Control-alt-F1, F2 etc had no effect.
Control-alt-backspace (!) had no effect. No disk activity.

Telnetted in from another machine and on 'top', X was
sitting at 99% of CPU fairly solidly. Lots and lots and
_lots_ of gettimeofday() requests zooming up the screen
with strace. Tried to shoot various apps to see if this
had any effect (nope) and then killed the X process: also
didn't work. Eventually shot it with kill SEGV. Want a
core file? :) (file reports it as a core of ld-linux.so.2
for some reason)

XFree86 package list.
[hobbit@aloss ~]$ rpm -qa | grep XFree
XFree86-SVGA-3.3.5-3
XFree86-VGA16-3.3.5-3
XFree86-xfs-3.3.5-3
XFree86-cyrillic-fonts-3.3.5-3
XFree86-libs-3.3.5-3
XFree86-XF86Setup-3.3.5-3
XFree86-3.3.5-3
XFree86-100dpi-fonts-3.3.5-3
XFree86-75dpi-fonts-3.3.5-3
XFree86-devel-3.3.5-3