What is happening is that we are the target of some hackers. They send a TON of requests at the site, trying to grab information from the databsse in some very clumsy, easily defeated ways.
But they never stop.
So what happens is that they sometimes fire off attacks back to back to back from different IP addresss, and do so very rapidly, causing a mini DDOS attack against the site.
I have set up one fence that sees their attacks, matches a pattern, then immediately drops the trafffic and kills them at the firewall level (thus preventing them from making any more requests).
But they change tactis from time to time, which can end up slowing the site down or knocking it - and the other webistes I run off of this server - off line.
I'll keep an eye on the logs and work to keep fencing them out.