Java APIs

Our system makes a large amount of HTTP requests via Java every day, and periodically we have periods where we get a large percentage of "java.net.UnknownHostException" exceptions (10-50%). Other times, the same URLs work fine, so we know the URLs are legitimate. Anyone have any ideas what can be going on? What situations produce this error?

We suspect it might have something to do with DNS rate limiting, but are not sure how to confirm this. Any thoughts on that as well?

We are using Java 6 and running on Windows 2003 servers.

Thank you.