I have a really strange SP 2010 issue I'd like to get to the bottom of. Occasionally, I will get a call in the morning that "Add document" link is missing, and the ribbon buttons are greyed out on a library. Typically this occurs for all libraries in all sites in a given web application. Recycling the app pool for the one web application fixes the issue, and it generally remains fixed for several days. I can't correlate with any particular ULS error or other event.
What could be causing this behavior, and what is the best fix?