That was exactly what I needed to know - likely a HW failure.

Did some testing with Prime95 and it revealed issues in the memory chain. Two sticks as installed produced a fail within a few minutes. One stick at a time, no problem. Two sticks swapped in the slots, rock solid on Prime95 and all other apps tried. I suspect the sticks weren't well seated and this fixed it.

Thanks Jephree for the super fast response and pointing me in the right direction! You da man.