Setting to 3600MHz might've fixed it. It defaults to "Auto" (which shows as 2133MHz). After I changed it to 3600, I went through a couple of shutdowns/startups without error. That didn't occur to me, for some reason. Will run with that for a couple of days and update here if problem occurs...
Specs at bottom.
When I power this PC on, I see lights on the motherboard and hear fans spinning, but no display. It'll just sit there in that state without any display or indication that it's managed to POST. If I hold the power button down until lights/fans all stop, give it a few seconds...