WHM/cPanel Server Optimization

I've been facing a problem with my server for 9 month now. It is always overloaded while I have a very good server. Here is the specification of the server: Quad Core Xeon® X3430 Processor 4 x 2.4 GHz Cores 8 MB L3 Cache, 1333 MHz FSB 4 GB 1333MHz DDR3 Memory 500 GB Usable Storage 6 TB Monthly Transfer 100 Mbps Uplink Port 4 IP Addresses CentOS 5 Operating System Unlimited Domains and User Accounts But I still get overload. I dont have much visitor and I am using 9 domains on the server. One of them uses wordpress article directory and another domain uses vbulletin and the other domains are either use very simple script or are closed. My server some times uses the whole resources that I have which makes me contact the server company to restart my server manually. I've tried everything and contacted the server company to resolve the problem but they told me that most of resource are used by MySQL. I think there is a problem with the wordpress database. I've tried to repair the database but nothing happened. I need a full optimization to my server and remove any unwanted temp files or emails or softwares installed in my server. Also I need this problem to be fixed permanently.. Thank you