Around noon today, my Web server became non-responsive. I remoted into the box to see that the CPU was pegged, all from the various Web serving app pools. Network traffic wasn't bad at all.
So I ran our old friend netstat just to see if there was any one IP that had excessive connections, and sure enough, there was one coming from China, hitting all of my IP's on port 80. Pretty weird stuff. I setup a security policy to block the entire subnet from China, and the problem went away.
I notified my host, The Planet, and they turned on some device that guards against traffic floods, from one source or many. I wonder if perhaps this had been going on for awhile, because it's a lot more responsive now. Very strange stuff. Very random too.