Stick to Bitdefender or kaspersky for server security, uninstall every program from the server that isn't a role and run the server COMPLETELY blank, disable internal security, and slowly bring features online to you narrow the issue down, replace cable from server and check that your swtiches are running STP, a user may be accidentally pluging a Eth back onto itself, causing the switch to flood, next time the server does it, logon and do \\servername\share if your able to access the share via the server name then the share is working, DO NOT USE \\LOCALHOST this bypasses alot of things, if your able to access the share locally then the server is working CORRECTY, and you must track back from the ethernet controller to the swtiches.