Website prompts for credentials

[Deb Silvia]

I have a Windows Server 2003 Web Edition server that is running Plesk 8.2. Websites on this server occasionally and random prompt anonymous users for credentials. When this happens Anonymous authentication is still enabled in IIS and no other authentication methods are set. Also, the file permissions do not appear to have been changed. The fix generally involves a combination of rebooting the server and restart the Plesk services.

I am not sure if this is an IIS issue or a Plesk control panel issue. Has anyone encoutered similar issues?

Thanks for any help!