[CentOS] Website running slow and Timing Out, but at random...

I have a site running the current version of Centos 4.2, it has a 
shopping cart running on it and runs slow and times out at random.  The 
cart is a simple php package using a MySQL database backend.

Can any one think of ways to improve the speed in MySQL and Apache, I've 
had the cart checked by the software vendor, it's working like it 
should, they are pointing to MySQL or Apache and a maybe...

Thank you,
John Plemons