Why does ubuntu server suddenly restart?

Question

Java

Good afternoon!
It so happened that my iron was released, which worked under Windows for a very long time and I decided to transfer ubuntu server to it. One of the components is a minecraft server. But the server started to suddenly restart in the interval from 2 minutes to 2 hours. I tried to turn off the minecraft server - the server has been standing for more than a day. And I would have given up on this minecraft, but in the future I plan to use no less heavy java applications here, and I don’t want the server to behave like this.
There is nothing in the logs about the moment of restart. The boot log just starts.
I tried memtest, badblocks and even burnP6 in 2 threads - the hardware is normal.
Configuration:
Run minecraft: su mc -c "screen -dmS gs /home/mc/s"
Java Version: 1.7.0_51 OpenJDK 64-Bit
OS: Ubuntu 12.04.4 LTS Proc
: Intel(R) Core(TM)2 Duo CPU E6750 @ 2.66GHz, 2 cores
Mat. Board: 4core1600p35-wifi with 4GB of RAM on board.
Please tell me what it could be and how to trace it. Thank you!

/var/log/syslog restart time - 15:14:18

Mar 8 13:31:07 smpv kernel: [ 16.899245] type=1400 audit(1394271067.774:24): apparmor="STATUS" operation="profile_replace" parent=859 profile="unconfined" name="/usr/lib/connman/scripts/dhclient-script" pid=864 comm="apparmor_parser"
Mar 8 13:31:07 smpv kernel: [ 16.899497] type=1400 audit(1394271067.774:25): apparmor="STATUS" operation="profile_replace" parent=859 profile="unconfined" name="/usr/lib/connman/scripts/dhclient-script" pid=864 comm="apparmor_parser"
Mar 8 13:31:07 smpv kernel: [ 17.053879] type=1400 audit(1394271067.930:26): apparmor="STATUS" operation="profile_load" parent=859 profile="unconfined" name="/usr/sbin/named" pid=866 comm="apparmor_parser"
Mar 8 13:31:07 smpv kernel: [ 17.060877] type=1400 audit(1394271067.938:27): apparmor="STATUS" operation="profile_load" parent=859 profile="unconfined" name="/usr/sbin/mysqld" pid=865 comm="apparmor_parser"
Mar 8 13:31:07 smpv kernel: [ 17.111123] type=1400 audit(1394271067.986:28): apparmor="STATUS" operation="profile_load" parent=859 profile="unconfined" name="/usr/sbin/tcpdump" pid=868 comm="apparmor_parser"
Mar 8 13:31:08 smpv cron[907]: (CRON) INFO (pidfile fd = 3)
Mar 8 13:31:08 smpv cron[914]: (CRON) STARTUP (fork ok)
Mar 8 13:31:08 smpv cron[914]: (CRON) INFO (Running @reboot jobs)
Mar 8 13:31:08 smpv kernel: [ 17.666310] type=1400 audit(1394271068.542:29): apparmor="STATUS" operation="profile_replace" parent=926 profile="unconfined" name="/usr/sbin/mysqld" pid=930 comm="apparmor_parser"
Mar 8 13:31:14 smpv dbus[718]: [system] Activating service name='org.freedesktop.ConsoleKit' (using servicehelper)
Mar 8 13:31:15 smpv dbus[718]: [system] Activating service name='org.freedesktop.PolicyKit1' (using servicehelper)
Mar 8 13:31:15 smpv /etc/mysql/debian-start[1167]: Upgrading MySQL tables if necessary.
Mar 8 13:31:15 smpv ntpdate[805]: adjust time server 91.189.89.199 offset -0.256145 sec
Mar 8 13:31:15 smpv polkitd[1154]: started daemon version 0.104 using authority implementation `local' version `0.104'
Mar 8 13:31:15 smpv dbus[718]: [system] Successfully activated service 'org.freedesktop.PolicyKit1'
Mar 8 13:31:15 smpv dbus[718]: [system] Successfully activated service 'org.freedesktop.ConsoleKit'
Mar 8 13:31:16 smpv /etc/mysql/debian-start[1170]: /usr/bin/mysql_upgrade: the '--basedir' option is always ignored
Mar 8 13:31:16 smpv /etc/mysql/debian-start[1170]: Looking for 'mysql' as: /usr/bin/mysql
Mar 8 13:31:16 smpv /etc/mysql/debian-start[1170]: Looking for 'mysqlcheck' as: /usr/bin/mysqlcheck
Mar 8 13:31:16 smpv /etc/mysql/debian-start[1170]: This installation of MySQL is already upgraded to 5.5.35, use --force if you still need to run mysql_upgrade
Mar 8 13:31:16 smpv /etc/mysql/debian-start[1230]: Checking for insecure root accounts.
Mar 8 13:31:16 smpv /etc/mysql/debian-start[1235]: Triggering myisam-recover for all MyISAM tables
Mar 8 13:39:01 smpv CRON[1571]: (root) CMD ( [ -x /usr/lib/php5/maxlifetime ] && [ -d /var/lib/php5 ] && find /var/lib/php5/ -depth -mindepth 1 -maxdepth 1 -type f -cmin +$(/usr/lib/php5/maxlifetime) ! -execdir fuser -s {} 2>/dev/null \; -delete)
Mar 8 13:52:15 smpv kernel: [ 1284.966215] perf samples too long (2503 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
Mar 8 14:09:01 smpv CRON[2103]: (root) CMD ( [ -x /usr/lib/php5/maxlifetime ] && [ -d /var/lib/php5 ] && find /var/lib/php5/ -depth -mindepth 1 -maxdepth 1 -type f -cmin +$(/usr/lib/php5/maxlifetime) ! -execdir fuser -s {} 2>/dev/null \; -delete)
Mar 8 14:17:01 smpv CRON[2258]: (root) CMD ( cd / && run-parts --report /etc/cron.hourly)
Mar 8 14:39:01 smpv CRON[2272]: (root) CMD ( [ -x /usr/lib/php5/maxlifetime ] && [ -d /var/lib/php5 ] && find /var/lib/php5/ -depth -mindepth 1 -maxdepth 1 -type f -cmin +$(/usr/lib/php5/maxlifetime) ! -execdir fuser -s {} 2>/dev/null \; -delete)
Mar 8 15:09:01 smpv CRON[2296]: (root) CMD ( [ -x /usr/lib/php5/maxlifetime ] && [ -d /var/lib/php5 ] && find /var/lib/php5/ -depth -mindepth 1 -maxdepth 1 -type f -cmin +$(/usr/lib/php5/maxlifetime) ! -execdir fuser -s {} 2>/dev/null \; -delete)
Mar 8 15:14:18 smpv kernel: imklog 5.8.6, log source = /proc/kmsg started.
Mar 8 15:14:18 smpv rsyslogd: [origin software="rsyslogd" swVersion="5.8.6" x-pid="703" x-info="www.rsyslog.com"] start
Mar 8 15:14:18 smpv rsyslogd: rsyslogd's groupid changed to 103
Mar 8 15:14:18 smpv rsyslogd: rsyslogd's userid changed to 101
Mar 8 15:14:18 smpv rsyslogd-2039: Could not open output pipe '/dev/xconsole' [try www.rsyslog.com/e/2039 ]
Mar 8 15:14:18 smpv kernel: [ 0.000000] Initializing cgroup subsys cpuset
Mar 8 15:14:18 smpv kernel: [ 0.000000] Initializing cgroup subsys cpu
Mar 8 15:14:18 smpv kernel: [ 0.000000] Initializing cgroup subsys cpuacct
Mar 8 15:14:18 smpv kernel: [ 0.000000] Linux version 3.11.0-18-generic ([email protected]) (gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) ) #32~precise1-Ubuntu SMP Thu Feb 20 17:52:10 UTC 2014 (Ubuntu 3.11.0-18.32~precise1-generic 3.11.10.4)
Mar 8 15:14:18 smpv kernel: [ 0.000000] Command line: root=UUID=47d24427-85f3-4148-8474-13be37732d39 ro quiet splash
Mar 8 15:14:18 smpv kernel: [ 0.000000] KERNEL supported cpus:
Mar 8 15:14:18 smpv kernel: [ 0.000000] Intel GenuineIntel
Mar 8 15:14:18 smpv kernel: [ 0.000000] AMD AuthenticAMD
Mar 8 15:14:18 smpv kernel: [ 0.000000] Centaur CentaurHauls
Mar 8 15:14:18 smpv kernel: [ 0.000000] e820: BIOS-provided physical RAM map:
Mar 8 15:14:18 smpv kernel: [ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x0000000000099bff] usable

Answer 1

close the server tightly with a firewall and start minecraft
Perhaps there are attacks, that's what it's about.
Put some munin ( munin-monitoring.org/)
See what and when it loads.
Again, the option with the load. Maybe there is a power drawdown on the PSU?

Answer 2

I have several assumptions:
1. The server is ddosed or bot-attacked, although they are almost the same thing.
2. Some kind of crooked plugin / mod that just cuts everything out.
3. As Calc said, the problem is nutrition.