Recently I have a huge problem. Some site stop working after a cpanel update ( we think ) .
Email client start to be slow, mysql give mysql server has gone away, some curl give us error.
After a few hour of research we find out that gethostbyaddr was the php function that kill the apps.
When we check our resolv.conf we see that one of the dns was working verry slow.
We exclude that, add google nameserver and everything was back to normal.
Also please check to not have your own server listed there.