Frequent prompt for username/Password on web site

I’m hosting a website. The site consists of a frameset with 2 frames. One frame is a link to a page that has a navigator. The other frame is a link to a document. The document is created using a very generic form. On the form is a Title field and a rich text field.Then there is a counter agent.

About 75% of the time when the site is accessed

it is promted for Username and Password authentication. Sometimes I can hit refresh or close down the browser and reopen and it will allow access. I have checked everything associated with the form and page and made sure there is no restrictions. I have default access in ACL at author and anonymous at author. I have also checked the server security docs to make sure the agent has access. I have even set a link to a different document (using same form)and it does not ask for username/pw. It appears to be this particular document. This document has some jpg images and very little text in the rich text field. I do not understand why it prompts for authentication occasionally. Any ideas?