For the first time ever I saw (from a remote telnet) the X processes CPU activity at 70%(ish).
The Matrix screen saver was running at the time and was giving me 1 frame a second (if that) neither mouse movement nor keyboard interaction would exit the screensaver (running as part of xscreensaver but not a package distrubed hack).
I killed the X process before it got any higher.
I then (anoyed) switched not only the AGP back to 4x in the bios but also selected all the AGP read/write accelorators I could find !
Hung 2 hours later, but since then has been behaving quite well.
I forgot to note before, if I have one of these hangs and then kill X the kdm restarts X (get the NVidia logo etc) and then any attempt to exit X is meet with a garbaled screen although typing (carefully) root password shutdown -r now works.