A sporadic 404 could be a sign server capacity is being maxed out. An average server will only be able to take 300-400 simultaneous connections for dynamic pages, probably thousands for static content. Once it's too much pressure the relief valve opens and it starts serving 404s . So if you hit the site at the same time as a couple hundred other people or the google bots you might get that. It's a timing issue which is why it's sporadic and hard to reproduce. At least that's my hypothesis.