Thanks, reddog. At least I have confirmation of similar behavior with a different DAW host. I doubt the hardware change accounts for the change in your case. (I'm running on a quad core i7 Mac Mini, BTW.) But maybe Mavericks makes a difference. I'm still on ML 10.8.5.
FWIW, this is what a friend of mine, who is a programmer at Apple, had to say:
"I suspect the DAW has closed a plugin, but the plugin still has a timer or other periodic task going on; that's usually a programming error on the part of the plugin writer (you're supposed to stop all timers and observations when you get closed)."
Just a hypothesis.