As it turns out, FRED's recent downtime was caused by an ill-behaved crawler run by a Chinese search engine. When this issue first arose, one of the first things I did was to look for excessive numbers of requests coming from single IPs, and this bot had been among the top 3 or 4 clients over the better part of that day. But because it made fewer requests than other crawlers such as Google, Bing, and Yahoo, I discounted it as a cause of the issue. After all, it had made fewer requests than those other well-behaved bots, which FRED has no problem serving.
However, I had retrospectively counted the Chinese requests over the whole day in aggregate. When I had a chance to watch the server processes escalate in real time, I saw that one IP address was making as many as 100 concurrent requests! It was the IP of that Chinese spider.
Adding a line to FRED's firewall config fixed the problem by blocking that IP (their whole class B subnet actually). So FRED's search rank in this chinese search engine will suffer, but I'm ok with that. :^)