Yes. Multiple times without noticeable improvement. Thanks for the suggestions. There may yet be something I have overlooked, but I'm beginning to doubt that. I just started up after having the machine off for four hours. It started right up. Sheesh! This seemingly irregular behavior is a pain.
Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5