There's not a lot to go on here - but the obvious pointer is, if there are eight windows servers, why is you access verified by a remote domain controller? You should certainly have one locally for the amount of infrastructure you have.
This feels a lot like a typical Windows DNS issue. If you run "ipconfig /all" on both the servers and the desktops, you should see your domain controllers in the DNS (and only the domain controllers, NOT "the domain controllers and my ISP"). Can you confirm this?