When I put the 280x back in...everything works just fine.
280x and a RX580 are made by AMD but they're different video cards. With different technologies, different power draw etc. that may be the reason the 580 isnt working, (whatever the differences are between the two may be what's causing the issue), increased power draw or something to that effect. Who knows ,maybe even your motherboard bios isn't compatible in its current version and you need to update to a newer motherboard bios.
I guess my point would be that ,if your issue doesn't show itself in a specific situation, it doesn't mean that the issue doesn't exist, it just means that the specific situation isn't triggering it. if i had a broken foot, and sat all day, I wouldnt feel any pain.....but if i were to get up and walk around, i would feel pain. Its not that there was no issue, its just that i wouldnt be triggering the issue to cause pain while sitting down. my guess is your 280x simply isnt triggering the problem