Verify that logon is not crossing a WAN link. If needed set up sites, subnets, and intersite links to control replication and logon traffic.
Verify that there are no invalid A, NS, and SRV records in your DNS.
Verify that all DC's are online and reachable.
Verify that every DC has shared its SYSVOL and that there are no errors in the FRS logs.
If using roaming profiles, verify that profile is less than 20 MB if pulling from local LAN server and less than 2 MB if pulling from server across a WAN link.
PSC
Governments and corporations need people like you and me. We are samurai. The keyboard cowboys. And all those other people out there who have no idea what's going on are the cattle. Mooo! --Mr. The Plague, from the movie "Hackers