The next few times this happens, could you record the exact date and time as well as the URL you were trying to access? We might be able to look at the server logs and see what's going on.

Edit: Never mind, I've looked at the error logs and found you. You're the only person I see getting errors like this. The error is "client denied by server configuration", whatever that means. You're not on any of our own ban lists, so we might have to discuss this with our hosting provider.