We're getting an intermittent problem with the homepage of our site when IIS will suddenly start offering to download the ASPX file instead of executing it. It happens about once a month, on a random basis. If I open the file in Notepad and re-save it (without making a single change to the code), IIS starts executing the script as normal and doesn't offer the download any more.

I can't get my head round this, considering that we can't reproduce the problem. Also, I can't really play with it and test anything when the problem occurs as this is the homepage of the site and needs fixing asap. Any ideas what could be causing this?

Files are written to the server over Filezilla FTP. Looks like the homepage is the only page affected. Server is Windows Server 2003, IIS 6.0 with all the latest patches (we actually run two of them, load balanced, and the problem can occur on either server on a random basis). Can't see anything in the Event logs or IIS log.