I have a potential client that has a php site that performs fine most of the time. However, every week or so, it will experience lag (slow page loading). I am sure there are a myriad of things that can be causing this (network issues, bad installation, a specific php file, increased traffic load). However, I need a way to deduce what is causing this. Is there any server monitoring software that is made especially to handle these situations?

PS: The server is linux