Randomly rebooting server

Randomly rebooting server

I have an older ubuntu server that reboots without help from me. All of the fans are running, and it doesn't seem to be overheating. Below is the output of syslog right before the reboot. As you can see in the last line, logging is stopped mid-stream.
Any ideas on what could be causing a hard reboot are appreciated.
May 19 10:22:04 test_server kernel: [   38.968782] init: plymouth-stop pre-start process (1678) terminated with status 1
May 19 10:23:11 test_server kernel: imklog 5.8.6, log source = /proc/kmsg started.
May 19 10:23:11 test_server rsyslogd: [origin software="rsyslogd" swVersion="5.8.6" x-pid="1010" x-info=http://www.rsyslog.com] start
May 19 10:23:11 test_server rsyslogd: rsyslogd's groupid changed to 103
May 19 10:23:11 test_server rsyslogd: rsyslogd's userid changed to 101
May 19 10:23:11 test_server rsyslogd-2039: Could not open output pipe '/dev/xconsole' [try http://www.rsyslog.com/e/2039 ]
May 19 10:23:11 test_server kernel: [    0.000000] Initializing cgroup subsys cpuset
May 19 10:23:11 test_server kernel: [    0.000000] Initializing cgroup subsys cpu
May 19 10:23:11 test_server kernel: [    0.000000] Linux version 3.2.0-39-generic-pae (buildd@aatxe) (gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) ) #62-Ubuntu SMP Wed Feb 27 22:25:11 UTC 2013 (Ubuntu 3.2.0-39.62-generic-pae 3.2.39)
May 19 10:23:11 test_server kernel: [    0.000000] KERNEL supported cpus:
May 19 10:23:11 test_server kernel: [    0.000000]   Intel GenuineIntel
May 19 10:23:11 test_server kernel: [    0.000000]   AMD AuthenticAMD
May 19 10:23:11 test_server kernel: [    0.000000]   NSC Geode by NSC
May 19 10:23:11 test_server kernel: [    0.000000]   Cyrix CyrixInstead
May 19 10:23:11 test_server kernel: [    0.000000]   Centaur CentaurHauls
May 19 10:23:11 test_server kernel: [    0.000000]   Transmeta GenuineTMx86
May 19 10:23:11 test_server kernel: [    0.000000]   Transmeta TransmetaCPU
May 19 10:23:11 test_server kernel: [    0.000000]   UMC UMC UMC UMC
May 19 10:23:11 test_server kernel: [    0.000000] BIOS-provided physical RAM map:
May 19 10:23:11 test_server kernel: [    0.000000]  BIOS-e820: 0000000000000000 - 000000000009f000 (usable)
May 19 10:23:11 test_server kernel: [    0.000000]  BIOS-e820: 000000000009f000 - 00000000000a0000 (reserved)
May 19 10:23:11 test_server kernel: [    0.000000]  BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
May 19 10:23:11 test_server kernel: [    0.000000]  BIOS-e820: 0000000000100000 - 000000002fff0000 (usable)
May 19 10:23:11 test_servMay 19 18:10:30 test_server kernel: imklog 5.8.6, log source = /proc/kmsg started.