ldap connection timing out

I have an asp.net version 4 application which is using windows authentication for security this works fine on a machine running iis 6 windows server 2003, but when migrating to iis 7 windows server 2008 r2 standard I get the following error:

System.DirectoryServices.Protocols.LdapConnection.ConstructResponse(Int32 messageId, LdapOperation operation, ResultAll resultType, TimeSpan requestTimeOut, Boolean exceptionOnTimeOut)

iis authentication is enabled for both windows and asp.net impersonation,开发者_Python百科 with the others being disabled (I have tried various combinations with no success). This is running in its own .net 4 app pool and I have tried both integrated and classic managed pipeline modes. From the event log I can see this code is causing the time out problem:

var searchResponse = (SearchResponse)connection.SendRequest(searchRequest); 

Am I missing a trick here?