[CentOS] Web server becoming unresponsive : high load

Hi,

I have a web server which is randomly becoming unresponsive. I've checked for hardware problems with no luck : - no ethernet errors - no disk errors - no memory errors

So far I've managed to point the trouble at the web server since if I restart httpd (apache) it comes back to normal.

This is not a shared server so I control (or at least like to think of) what is hosted so I have two suspects: a) my own code is behaving strangely. For example, a php script that in a given situation/parameters increases the usage. b) some sort of attack. Either in conjunction to a or other vulnerability.

I am looking for advices in how can I pin point since looking at every line of code would be infeasible.

Centos 5.5 / apache / php 5.2.10