Nslookup Times Out on A Lookup To Well-Known Hosts

[Will]

I have several well-known hosts that are failing A record lookups on DNS and
I need help debugging this.

We have the ISC Bind 9.3.0 set up on a box with a sendmail mail server, just
to speed up the MX lookups on that box.   I go to the nslookup command line,
and type something like the following:

    type=mx
    cox.net

and what I get back is a timeout that lasts two seconds and then no
response.

I start a sniffer, and record while performing the above.   There is no
question that the name server is issuing the query, and that the remote DNS
responds with the nameserver (NS) records for cox.net.

At this point, the sniffer trace shows that there is a timeout on simple
Address (A) record lookups to all nameservers for cox.net.

I confirm that result from the command line by simple nslookup to ns.cox.net
(for example) and this does time out.

I jump over to a UNIX box outside our network and try the same queries, and
they all immediately work.

What are some possible causes for this?    Could cox.net be blacklisting
many Internet hosts on their nameservers?

-- 
Will