Heavily-loaded MySQL: Resource temporarily unavavailable?

[Chris Boot]

All,

I'm running a loaded MySQL server and every so often MySQL seems to refuse
connections in batches, which manifests itself as the following errors in PHP:

mysql_connect() [<a href='function.mysql-connect'>function.mysql-connect</a>]:
Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock'
(11) [path to file] on line [x]

I've got a carefully tuned my.ini and Apache configuration, which can
theoretically process 400 connections at a time and this works most of the time.
MySQL is set to accept 600 simultaneous connections though this is never reached
(according to phpMyAdmin's stats anyway). I've upped the open files limit on
MySQL and Apache (PHP runs as a module).

At this stage I'm completely out of ideas as to what I can do to fix my problem.
Any ideas? What extra information can I provide that could help?

Many thanks,
Chris

-- 
Chris Boot
bootc@stripped
http://www.bootc.net/