I had a problem like this once on an old 7100 or 7200 (I can't rember which -- it was a nubus machine with an 80Mhz 601 PPC I think) and it turned out that xscreensaver was causing the X display server to crash. The nice thing was that xscreensaver was nice enough to give some message when I ran it that said something like "you must disable the X server's built in screensaver first" and it even gave the proper incantation. Once I did what it said, xscreensaver ran just fine and X was really stable.
Hope this helps.
