Server randomly stops responding

[Jayock]

I am running an FC7 server setup using the Perfect server guide, and running ISPConfig. The server randomly stops responding to all requests except for telnet, which always goes through. This happens fairly frequently with dovecot, apache, and SSH. Restarting the services will typically bring them back to life, but not always. They go back down again, and I repeat while I try to figure out the permanent solutions. Nothing in the system logs seems to be showing me any solutions. Any ideas where to start looking from here?

Im wondering if it might be a PAM issue, or PHP scripts on my website (which worked perfectly on a debian server before migrating here).

Thanks,
Justin