[CentOS] gdm won't start

I'm trying to set up a new CentOS 4.2 desktop, and gdm refuses to
start.  It blanks the screen, displays the swirling blue circle mouse
cursor that appears to be an hourglass equivalent, then drops back to
the console window.  It repeats this several times then displays the
following error:

The display server has been shut down about 6 times in the last 90
seconds, it is likely that something bad is going on.  I will wait for
2 minutes before trying again on display :0.

xdm works, and I can access X Windows by running startx.

Any ideas on how to troubleshoot?  It's a fresh install with a few
site-specific customizations, but I don't think that I've done
anything that would intefere with gdm.  It is a relatively minimal
install, so it's possible that I'm missing some needed package.

Thanks.

Josh Kelley