ASP.NET Error: cannot load assembly

I often had this problem with ASP.NET pages:

Configuration Error: cannot load assembly 'assemblyname.dll'

the error occurs in machine.config on line:

The problem would appear out of the blue and last for a while, after which it would "fix itself."

I finally found the cause and solution:

1. temporary solution (to get your application to work again): stop and restart the indexing service

2. permanent solution: disable indexing on the ASP.NET temporary folder

Here's more info:
http://support.microsoft.com/default.aspx?scid=kb;en-us;329065