Server extremely slow Postfix not working

I am running ISPCONFIG with Fedora Core 5. It's been running flawlessly for over a year now. All of a sudden I can't send or receive any mail. When I reboot the server it takes an extremely long time to come up to the login screen.

On a side note. I can reload everything from backups, reboot and it works great for a couple of days and then it becomes unstable again.

Any help would be greatly appreciated.

 

Any errors in the logs ?

 

Which log files should I check. Sorry, I'm not to familiar where things are on this system.

Thanks

 

/var/log/maillog
/var/log/messages
/var/log/secure
/var/log/audit/audit.log