Thank you for the response, and yes, that is my issue. The FSPX signal up from the PPU to the controller isn’t there. Or, more accurately, flickers in and out at points.

That’s really what I am trying to figure out. What does the PPU need to see to send that signal? I’ve tried swapping around some of the I/O cards inside the PPU and it actually does have some effect on the frequency of the flickering. But I’m not sure if that’s dumb luck, or if it’s a problem with the bottom motherboard, or if there is some other signal the PPU needs to first see to send fspx.