gdm restarts every 20 minutes: why?

[Nick Jacobs <halbtaxabo-debuser@yahoo.com>]

If there is no user activity, gdm restarts every
20 minutes, killing the current local session. Here's
a sample from /var/log/syslog.

Mar  5 07:05:27 leo gdm[8899]:
gdm_slave_xioerror_handler: Fatal X error - Restarting
:0
Mar  5 07:25:29 leo gdm[8912]:
gdm_slave_xioerror_handler: Fatal X error - Restarting
:0
Mar  5 07:45:30 leo gdm[8932]:
gdm_slave_xioerror_handler: Fatal X error - Restarting
:0
Mar  5 08:05:32 leo gdm[8945]:
gdm_slave_xioerror_handler: Fatal X error - Restarting
:0
Mar  5 08:25:34 leo gdm[8958]:
gdm_slave_xioerror_handler: Fatal X error - Restarting
:0

There's nothing in /etc/cron.d with a period of 20
minutes. There's exim, which is set to run every 15
minutes, and it does run every 15 minutes. The
directories under
/var/spool/cron are empty. I'm not running a
screensaver.
I'm running Woody, with the latest updates.
The kernel is 2.4.18-bf2.4

Any ideas?