Credentials Prompting

We recently migrated an app from IIS5 to IIS6. One strange thing is
periodically occuring. Some users are getting prompted for credentials in
areas of the app when they are already authenticated. This is a windows
authenticated app. It has not rhyme/reason. Different users, different pages,
different data within the pages, different offices. On potential commonality
is the sluggishness of the page just before the prompt. Does anyone have any
clue?

Thanks in advance,
Jim