I have been facing mysterious server hangs recently, and it doesn't always happen during high usage, the thing could die without anything touching it, or it could survive a peak usage lunch break.
When it died, it just died, ping timeout, the console frozen, even the crontabbed auto-reboot script failed to execute.
A reboot will fix it but it will come back again.
Recently I have got another rig and swapped in one of the server which has been hanging like hell, it has been running for about a week till now without any issue(at least from my point of view).
Therefore, I too agree with Peter, if it is something wrong, check the a) network, b) hardware.
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Check out the latest post in my thread: http://forum.pfsense.org/index.php/topic,34563.0.html