Troubleshoot server freezing randomly in middle of night

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http://www.linuxquestions.org – OK, I run a server but not ubuntu server. I run Lubuntu 12.10 desktop on it. I then run 2 virtual machines with 1gb ram each. I then run crashplan. Each night I run a backintime cron job backup as I have almost 2 million files and 2 x 3tb hard drives. One backs up to the other in this cronjob. Just randomly about 2-3 days ago the server froze. It has not frozen ever since I installed 12.10 on it. Then it froze again last night. Here is part of my syslog file which seems to indicate it is out of memory. The server has 6gb of ram in it. Code: Dec 17 01:17:02 lubuntu-server kernel: [227133 (HowTos)