Wordpress and Systems admin needed to correct issues on server

Hi, I have a cluster of servers that run about 15 wordpress sites and receive a fairly large amount of traffic. Recently several sites have become un-responsive or respond occasionally. This has dropped our rankings in google. We are needing someone to come in and discover the issues that are causing this and correct them. We are not sure if the issues are related to wordpress or a specific configuration on the server. Please respond with ONLY your experience with Linux and Wordpress. Thanks and good luck