many guys come to Microsoft newsgroups with AD related problems.
Many of them could be avoided by following these advices:
- never get more than 5 minutes clock offset between any computer in the domain (and the one you are going to put in the domain)
- Always have your domain members (servers and workstations)using you DC's DNS
- DC should point to themselves as DNS, not your ISP
- Operation Master FSMO role can't be on a Global Catalog, except if you have only one DC or if all DC are Global Catalog
Hope it helps,