How can I diagnose an Ubuntu system freeze after reboot
Solution 1:
My first action would be to take server out of service and run a full Memtest+ run on it to check that memory is not failing. Next check SMART from HDD's for any issues. Next would be to following instructions at http://www.kernel.org/doc/Documentation/networking/netconsole.txt to capture anything like this in future.
Solution 2:
Im guessing you've seen this already but how-can-i-use-syslog-to-diagnose-mysterious-crashes dont know if this could help at all, is your servers under stress/serve a great deal of clients