[ALUG] System shutting itself down but I can't see why

My Slackware 11 keeps shutting itself down but I can't see why. In /var/log/syslog and /var/log/messages there are messages from apache and samba saying they're shutting themselves down because they've been told to but I can't see a reason anywhere. It happens fairly randomly anywhere from once every ten minutes to once every several hours. It reboots all by itself and continues running. It's not a temperature problem as the system is way below the BIOS alarm temperatures. Any ideas as to how to diagnose this or places to look? -- Chris Green (chris@halon.org.uk)