what happens if you use the 780 ti in the other gpu slot .
tried that, pc then refused to turn on. I also noticed that tapping the side of my pc turns it on. I just purchased a new 850w single rail psu for testing purposes, pc still shuts down. At this point, I'm lost. Maybe there's a motherboard contact issue? i just don't know.
edit: it no longer just shuts down under load. it shuts down a short while after hitting the desktop.