I was having issues with my repoze.zope2 and Deliverance setup locking up on me. Turns out, since I had some of sub-requests in my Deliverance rules, it was occasionally causing a race condition where all the wsgi server threads were taken up and an existing thread was making a sub-request.
The fix is to remove all sub-requests in the deliverance rules file if possible, add more threads threads in the repoze.zope2 wsgi server configuration, or use a different wsgi server that doesn't restrict the amount of threads--so it'll add a new thread if needed. A better solution, IMO, would be to just have a different server, serve the theme files. Anything to make it not make sub-requests against the same wsgi server.