web app randomly prompts for NTLM authentication

I have an ASP.Net web application that I recently updated to framework 4.0. I had to install the new framework on the server that was running the app in production as well. The server is a 32 bit Windows 2003 Standard Edition running IIS 6. The application is using Windows authentication.

I say that the prompt is random as I have see开发者_StackOverflow中文版n it in two different places under different circumstance.

Any ideas here?