i am having the same issue with Jaunty. (9.04) Gdm spontaneously restarts (randomly) and not related to any other software. here is my sys.log 
May 11 16:32:50  syslogd 1.5.0#5ubuntu3: restart.
May 11 16:32:50  anacron[3149]: Job `cron.daily' terminated
May 11 16:32:50  anacron[3149]: Normal exit (1 job run)
May 11 16:40:01  /USR/SBiN/CRON[6108]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 16:50:01  /USR/SBiN/CRON[6348]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 17:00:01  /USR/SBiN/CRON[6583]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd hourly 2>/dev/null)
May 11 17:00:01 behlul-laptop /USR/SBiN/CRON[6587]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 17:10:01 behlul-laptop /USR/SBiN/CRON[6855]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 17:17:01 behlul-laptop /USR/SBiN/CRON[7173]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
May 11 17:20:01 behlul-laptop /USR/SBiN/CRON[7229]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 17:30:01 behlul-laptop /USR/SBiN/CRON[7466]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 17:40:01 behlul-laptop /USR/SBiN/CRON[7711]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 17:50:01 behlul-laptop /USR/SBiN/CRON[7943]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 18:00:01 behlul-laptop /USR/SBiN/CRON[8172]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd hourly 2>/dev/null)
May 11 18:00:01 behlul-laptop /USR/SBiN/CRON[8173]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 18:06:03 behlul-laptop gdm[4204]: WARNING: gdm_slave_xioerror_handler: Fatal X error - :0 restarting
May 11 18:06:04 behlul-laptop bonobo-activation-server (behlul-8360): could not associate with desktop session: Failed to connect to socket /tmp/dbus-GaHxEml4kX: Connection refused
May 11 18:06:05 behlul-laptop acpid: client 4216[0:0] has disconnected 
May 11 18:06:05 behlul-laptop acpid: client 4216[0:0] has disconnected 
May 11 18:06:05 behlul-laptop acpid: client connected from 8381[0:0] 
May 11 18:06:08 behlul-laptop acpid: client connected from 8381[0:0] 
May 11 18:06:15 behlul-laptop pulseaudio[8491]: pid.c: Stale PID file, overwriting.
May 11 18:06:18 behlul-laptop gnome-session[8419]: WARNING: Could not launch application 'nm-applet.desktop': Unable to start application: "nm-applet" oğul süreç çalıştırılırken hata oluştu (No such file or directory) 
May 11 18:06:18 behlul-laptop pulseaudio[8491]: module-x11-xsmp.c: X11 session manager not running.
May 11 18:06:18 behlul-laptop pulseaudio[8491]: module.c: Failed to load  module "module-x11-xsmp" (argument: ""): initialization failed.
May 11 18:06:19 behlul-laptop anacron[8690]: Anacron 2.3 started on 2009-05-11
May 11 18:06:19 behlul-laptop anacron[8690]: Normal exit (0 jobs run)
May 11 18:10:01 behlul-laptop /USR/SBiN/CRON[8827]: (root) CMD ([ -x /usr/sbin/update-motd ] && /usr/sbin/update-motd 2>/dev/null)
May 11 18:10:38 behlul-laptop gdm[8364]: WARNING: gdm_slave_xioerror_handler: Ölümcül X hatası - :0 yeniden başlatılıyor 
May 11 18:10:40 behlul-laptop acpid: client 8381[0:0] has disconnected 
May 11 18:10:40 behlul-laptop acpid: client 8381[0:0] has disconnected 
May 11 18:10:40 behlul-laptop acpid: client connected from 8933[0:0] 
May 11 18:10:43 behlul-laptop acpid: client connected from 8933[0:0] 
May 11 18:10:54 behlul-laptop pulseaudio[9044]: pid.c: Stale PID file, overwriting.
May 11 18:10:58 behlul-laptop gnome-session[8971]: WARNING: Could not launch application 'nm-applet.desktop': Unable to start application: "nm-applet" oğul süreç çalıştırılırken hata oluştu (No such file or directory) 
May 11 18:10:58 behlul-laptop pulseaudio[9044]: module-x11-xsmp.c: X11 session manager not running.
May 11 18:10:58 behlul-laptop pulseaudio[9044]: module.c: Failed to load  module "module-x11-xsmp" (argument: ""): initialization failed.
May 11 18:10:59 behlul-laptop anacron[9216]: Anacron 2.3 started on 2009-05-11
May 11 18:11:00 behlul-laptop anacron[9216]: Normal exit (0 jobs run)