I thought I had run Windows Memtest and Whatever the latest standalone Memtest86 clone there was at the time, when I jumped from 64gb to 128gb and it was fine. Only thing I can think of is maybe a bios update broke compatibility in some unforeseen way? I've seen comments floating around that Memtest is not entirely perfect, in part because the control schemes in DDR have gotten more and more complex since memtest came out, if I'm even remembering that right, so there can still be outliers who test fine but are plagued by infrequent random errors.
What was it in Event Viewer that you were seeing?
A friend of mine suggested BluescreenView and to check Event Viewer. The problem is whatever is causing the black screen shutdown is NOT the result of a bluescreen. Event viewer just shows generic power related messaging and searching the web for what I saw was a waste of time. There was nothing that looked valid enough to be worth the extended dive into the rabbit hole.
I bought a new Seasonic Vertex PX-1200w power supply this past February, which did replace a Seasonic SS-860XP that was older than I thought when I did the swap. Yet I saw ZERO change in overall system stability. The power tables are damn near identical except for the 12v on the newer one being 1200w instead of 852w. There was a minor downgrade on the newer model's -12v rail only being 0.3A instead of 0.5 on the old.
So either my old PSU was getting weaker in a way that the GPU's components couldn't cope with the sag in supply or AMD's recommended PSU minimum is absolute bullshit and these cards need 1500w to handle draw spikes.. 🤦♂️