DNS DoS ???

my DNS servers were getting slow so I blocked recursive queries for all but my own network. Then I was getting so many of these: ns2 named[5056]: client 78.159.111.190#25345: query (cache) 'isc.org/ANY/IN' denied that is was still slowing things down. I've since written a script to watch the log and throw these into the box local firewall. If I expire the entries after 24 hours then I accumulate about 10200 unique IPs. If I expire after 48 hours, then it's just over 20000 unique IPs. Is anyone else seeing this? Elliot