Every DNS server should have it's Network Card's DNS server pointing to itself. Not another server, or multiple IP's.
I've got 4 domain controllers (lets call them Srv1 Srv2 Srv3 and Mail) - all with DNS istalled (Active Directory Integrated). DNS settings within TCP/IP settings on Srv1 & Srv2 point to each other; DNS settings within TCP/IP settings on Srv3 & Mail point to Srv1 and Srv2. I thought that if a DC was running DNS, the TCP/IP settings for each within network settings should have ALL DC's IP addresses listed? (Just read that back and not sure if it makes sense??!!).