[CentOS] Server reboots unexpectebly.

Dear group,
I am in the process of configuring 2 new servers. They are running Centos 5.5 and for the last three days they have been rebooting unexpectedly, can you point me in the right direction what to look for in the logs. I have been checking /var/log/messages but don't see anything that hint me any clues why this is happening. Your input is much much appreciated. Lisandro 

PS. These are custom built Tyan boxes.

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