And what is your max concurrency for the webserver itself? This may help you.
http://httpd.apache.org/docs/2.2/mod...tml#maxclients
If it is just certain pages, what do they all have in common? Can you post your mysql config? Taking a stab in the dark here, I'd say if the apache.conf is correct and not exceeding, then it's most likely a MySQL issue and possibly a PHP timeout issue.