DNS behave abnormal

I have a DNS server on windows 2000. It behaves
abnormal sometimes. What it does is that it stops
responding after a while. People can get to certain websites but not all. Then I restart the
DNS services and everything works fine. Any suggestions what would be causing it.There are no error messages or anything. We are using AD as well and are main DNS are hosted by our ISP. This is just a dns that route stuff.

Thanks.