Yet another mystery

I am bedeviled by an intermittent fault when I run our web app from an IIS virtual directory on my own workstation (Win 2003 Server). Sporadically, the site map object won't get initialized, the result being that an exception gets thrown, with the associated error message:

The file web.sitemap required by XmlSiteMapProvider does not exist.

Needless to say, the file does exist and is accessible—so nearly all of the forum postings against this error message aren't very helpful. I don't have this problem when I run the app using the web server built into Visual Studio. A solution rebuild seems to make the problem go away for a short while.

I will post again if I ever figure out this problem.