Everything seems more or less normal now, but I'm going to leave MaxClients down around 100 for a little while, just to see how it goes. In an hour or two, if all is well, I'll set it back up to 150.
150 is the default, and generally not the best choice in my experience. On Bomis, we usually set it much higher, but only after a fair amount of experimenting with memory usage, etc. Possibly the 150 clients setting is a performance bottleneck on most days.
--Jimbo