Occasionally, I get a complete machine lock (can't change terminals, etc)
in xscreensaver. This happens completely at random, but afterwards
requires a reboot. If I disable the screensaver, the machine never hangs.
What graphics card and X server are you using?
I have a very similar problem. The screensaver crashes and completely locks
the station, or returns to gdm. The X server randomly crashes too (I cannot
change terminal but I can make a telnet from another station).
I search the source of this problem, and I think that
the problem come from the PCI bus gestion in the kernel (when
X freezes and when I make 'init 3' by telnet, I turn my monitor
off because the video card doesn't send anymore signal).
I have an Elsa Gloria Synergy 8Mb on a Compaq XP900 
(RH 6.2 and I have tried the kernels 2.2.13/14/15, XFree86-3DLabs 3.3.6)
with 640 Mb of RAM and 1 Gb of swap.
Have you tried any of the XFree86 *4* kits?
This is a bit old, don't ya think? :)
I'm now running 7.0 and it had similar issues, but there's a separate ticket
logged (and resolved) on that. Seems to be a driver issue and an update fixed