DirectX 9 mode doesn't appear to have any crash/instability, although it still looks terrible [shadows on things ahead of me waver as I move toward them, for example].

As far as I know, absolutely nothing changed on the system between Saturday afternoon and Sunday morning.

DxDiag doesn't show any problems - it's using DirectX 12 with an NVIDIA GeForce GTX 1060 with 6 GB video memory. I've had the same setup for almost a year at this point.

I do use a second monitor.