I saw this thread created yesterday and thought that you would get responses from people who know far more about web servers than me. However, one day later and no response so here goes.
I've installed and manipulated the parameters of Apache and PHP many times and found the parameters set can make a whole load of difference to performance and other buggy issues. I would suggest that you start to launch your own investigation by adjusting some of the parameters.
A quick search found these two discussions which may be of interest: