Glad to have found this thread again!
I have the same Gigabyte mobo and have run into some issues recently. I moved in February and just finally tried to power up the machine. Only once did it boot into my Sierra partition for maybe 2 minutes, then it shut down. When I tried to power it up again, it would go into that loop of starting for a few seconds, shutting down, start again etc.
I removed everything (PCI cards, extra drives) leaving only the main SSD and connecting my monitor to the internal HDMI which I've been using since the beginning. Didn't make a difference as far as the boot loop issue. I removed all 4 ram sticks and walked them one by one in each slot. Only once did the machine fully boot (with 1 stick), but soon the same thing happened as far as the reboot loop.
Decided to switch the motherboard BIOS switches to setting 2 (both switches) Now the machine stays booted, but I have no video. I was out of ideas, so I took the machine to a local PC Central to have them check it out. Now thinking about it, I wonder if when I changed the BIOS switches, it changed the BIOS settings to use one of the PCI slots for video. Does anyone know if that's the default setting? I was kind of exhausted with the whole thing and didn't think to try and get into the BIOS settings to verify anything.
Does it sound like the original BIOS got corrupted and I might be able to use the 2nd one, restore the first etc? I'm pretty sure it's not the PSU because the computer stayed running after changing the switches to position 2. Has anyone else had a similar issue? The machine worked great since I first built it until now. Maybe something happened when I moved as far as the case getting shaken etc??
Thanks!