It might seem obvious, but environmental problems in server rooms and wiring closets can have a deleterious affect on overall network performance. Hot servers aren’t happy servers, and the same goes for network hardware. Make sure your datacenter isn’t making the mercury sing, and install environmental sensors to keep it that way.
On a related note, it’s always a good idea to leverage the server-management utilities that may have come with your servers. Dell’s OpenManage and HP’s Insight Manager provide very pertinent information regarding the health and well-being of your server infrastructure, including whether or not it has a fever.