Cool deal finding the issue. Do you use any temp monitors/loggers? If not, I recommend CPUID HW monitor for overall system monitoring, or good ole MSI afterburner for GPU monitoring, but I use Asus GPU Tweak III since I have an Asus card. I used the latter to find and log voltage spikes/drops before shutdowns in MSFS. Weirdly, the on Seasonic PSU I had worked fine for everything but FS. In fact, it’s now powering another computer here without issue.