For me it was flash the bios using USB, then I can boot it once in every X attempts, switching off the power completely before trying again.
Be sure to let the bios flash correctly before you unpower 😉
If it then boots to code 78, you probably have the same problem as I do.
If you reboot at it complains about 'overclocking failed', then it's identical.
I opted for replacement as well, after the initial troubles with this motherboard I had a good almost 2 years with it.
I guess there is a fundamental design flaw in there somewhere, because there is no clear cause.
Resetting the BIOS should always use stock settings and so result in a boot, perhaps asus can check and perhaps fix any settings that are not 'factory standard'.
The system runs fine after boot, for the past few months sometimes it would fail to boot windows (something about watchdog).
Rebooting always solved it, so I figured it was a windows thing.
CPU is set to AUTO levelup, perhaps this is the overclocking that fails?
78 is ACPI, unsure what might cause it to get stuck there.
Reflash, clear CMOS, boot, if stuck: switch off, wait 10 seconds, switch on, boot, set bios settings, reboot (gets stuck), switch off, wait, switch on, boot (overclocking failed), reset and it should finally boot.
So yeah leaving this system on until the replacement comes. No more asus mobo for me... 😞