If you are using statically assigned addresses, this should maintain your DNS order. All machines in the domain should point ONLY to the DNS server on one of the DCs, with the secondary being pointed to a secondary DCs DNS service. The DNS service on the DCs should be set up to forward requests to the ISP DNS servers.
What does an IPCONFIG show for DNS order?
If this is the way you are set up, and you are getting responses from other than your local DNS, I would look at the hosts file on on the local machine. Also do a search to be sure that there are not multiple hosts files. (should only be one in system32driversetc and in i386 or service pack stores) if you have more than one suspect a hijack, although I aould have thought that it would be doing it all the time if that were the case.