High CPU usage and server load using Codeigniter

[eluser]fouadami[/eluser]
Hello,
I'm using codeigniter for a web game and I'm facing with a high server load right now. The game has about 300,000 visitors per day. When I 'top' in shell, mysql usage is about 2% to 30%, but the avg is 5% which is somehow normal for the game but httpd and php usage is very hight. Sometimes an httpd command takes more than 15% cpu and PHP also takes 6-7% in the peak.
I've upgraded Apache to the latest stable version but It doesn't help at all. Is there anything wrong with my server/script/codeigniter?

Thanks