Windows 2003 Server, Windows XP pro clients IE 7.0

We have a MOSS 2007 farm set up with Kerberos security. It works great
99% of the time. However, there is an odd quirk.. every now and then,
and in the middle of any given operation or any time following logging
in.. can be minutes after loging in or even hours... Sharepoint will
prompt for authentication, you can attempt to enter your credentials,
but it does not let you in. If we clip the web address line, close the
IE browser, reopen a browser and paste the address line, the session
continues with no prompt for credentials.

What could this be?

Thanks for any help or information.