Also, are you showing any errors in your ACP server error log or the actual HTTP server error log.
What hosting company are you with, and are you on a shared, VPS, or dedicated server. If shared, you may be on an oversold node (same if you are on a cheaper OpenVZ VPS).
What version of mysql are you using?
What version of PHP?
What HTTP server are you using (Apache2, nginx, LightSpeed).