I would start by turning off TCP offloading. There have been a few things that cause intermittent connectivity issues and this is the one that is usually the culprit.
Note: I have seen this setting cause problems on Win Server 2003 and Win Server 2008
http://blogs.msdn.com/b/mssqlisv/archive/2008/05/27/sql-server-intermittent-connectivity-issue.aspx
http://technet.microsoft.com/en-us/library/gg162682(v=ws.10).aspx