Named Stop Response Periodically

[bind at guangbin.interland.net]

Could anyone tell me what's wrong with the name server?

I'm using a RedHat Linux box to run bind 8.1.2 and there are thousands of
zones in my name server. I found that every about 6 minutes, the name server 
will stop response for 1-2 minutes. During the freezing period, the named
didn't process any name query and I can't start nslookup at local machine.
After 1-2 minutes, everything is normal.

This kind of periodical stopping response of named has last quite a long
time. What's wrong with my named?

Thank you very much!

G Liu