I would also have to say I used to have a problem when loading AD where it would take aaages to load, in the end it was a DNS problem pluss LDAP but then it was always down to missconfigured DNS in the end. So long as all IP's are registered in the DNS and both forward & reverse zones are all there then all should be fine, test using NSLOOKUP, if there's an error for the root server then it's missconfigured.

Or could always be something else.