I called AppleCare, but they blamed everything except the Mac. I work in video graphics, so have a lot of extra stuff on the machine. It's impossible for me to spend any time working without the various boards/drives/drivers installed. As it's happening less often now, I'll just put up with it until the 970's out.
And I spoke to a guy at the genius bar in Apple's SoHo store. He suggested tightening the fan's screws.