As an educated guess I'd say either there has been a power spike/overload or PSU failure that puts it into protection mode or there is something going on with your mainboard.
When you say "The Gigabyte board asks me to pick the last good boot" what exactly is the message ? Is it falling back to a redundant BIOS image ?
Is the PSU adequately sized for the machine..I know we had this conversation late last year on the list. But specifically if you use a PSU calculator for your combination of hardware components does the current draw on the 12v rails come in at way under whatever it says for that rail on the PSU spec.
If you go by wattage alone you don't know if the distribution of capacity is sufficient across the rails for your hardware. You often can't for example draw the total rated capacity from one rail. Sometimes despite the overall rating appearing to be sufficient the 12V rail can be undersized, this is particularly true on PSU's designed for older hardware.