You should definitely take a look at /var/log/messages as suggested above. In addition, is this a (semi-)managed server by any chance? If so, you can probably ask the techs to take a look at the issue for you as well.
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Odds are the problem is with Apache or mySQL. Your Apche errors logs might be helpful but for the most part, once a reboot is done you lose most all of any useful information, that help determine the exact problem.
Probable solutions are; add more more RAM and/or optimize Apache and/or mySQL.
Last resort is to get a beefier Server or another Server, to transfer some accounts onto.
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Ask your technicians for assistance... believe me, they don't *WANT* to have to go into the server room and manually reboot your machine when it crashes, so it's in their best interest to help you.
Cory von Wallenstein, Dynamic Network Services Inc.
My team and I spent the last few months putting together the DynDNS Spring Server VPS platform for folks needing IPv4 and IPv6 Linux servers on Xen. Would love feedback, both good and bad!