DNS request - reply from unexpected source

Hi!

My PROBLEM: Client send request to public DNS, but get answer from other
server and in this case the client reject the data. Client cannot resolve
DOMAIN NAME to IP.

INFO:
We have two public DNS servers (SERVER_IP_1, SERVER_IP_2) which resolve all
DNS requests and one other (SERVER_IP_3) server on that is our internal DNS
server. This servers has been peviously configured with Network Load
Balancing. The NLB was removed, and now only the two DNS servers should
resolve DNS requests. But mostly after DNS get Event-ID = 5504 & 3000. The
request are answered from the third server SERVER_IP_3 and not from the
server where the request was sent to. This is a strange behavior.

On Mac OS X the nslookup report:
"reply from unexpected source: SERVER_IP_3#62450, expected SERVER_IP_1#53"

This is visible also in Ethereal/Wireshark network messages.

Thanks for any input!