Corrupt bios?

Mobo: Gigabyte GA-Z77-D3H (Rev 1.0)
CPU: i5 3570k
GPU: Asus 970
Ram: 8 GB DDR3 G.Skill
PSU: EVGA Supernova G2 850 W
SSD: SanDisk Ultra Plus HD: WD Blue 1 TB
OS: Windows 10

Build had been working fine day before, and I have used this same hardware configuration for a little over a year.

When I come home on Monday, I was surprised to find my computer booting up to a frozen BIOS splash screen (would not register inputs from my PS/2 keyboard). Occasionally, it wouldn't even boot to that, but the computer would restart itself several times (power down after a few seconds, restart itself again, and then maybe make it to the splash screen).

I removed my GPU and plugged into integrated graphics. I also turned off the computer, cleared CMOS with the jumpers, and held the power button down for 10 seconds (which turned the computer on and then off), then booted into a screen that said my main BIOS has been corrupted and would restore from backup. At this point, using QFlash, I updated BIOS (for GPU compatibility) and powered down to re-insert my GPU. Rebooted, everything seemed to be fine. I thought maybe my troubles were over. Unfortunately not.

After powering down my computer and coming back later, I had the same problem as before. Frozen BIOS and my computer repeatedly restarting itself. I removed GPU, same steps as before, restored the backup BIOS over my corrupt main BIOS, and this time I left my GPU out. I shutdown computer for a few minutes and came back to yet another corrupt BIOS.

I've probably restored from the backup BIOS five times by now. It gets me into Windows where everything seems to work OK. Even when I flash it to the current version and restart, the computer runs well. But there is something about powering down that's causing this issue. I've unplugged and re-plugged PSU cables, reseated my mobo, tried alternating my RAM sticks, cleared CMOS with jumpers and battery, tried flashing to different versions of BIOS...but every time, powering down the computer results in this corrupted bios problem.

The only thing I can think is that on Saturday night, I opened the case to clean it out and tighten some screws. But it worked fine that night and the following day. Do I need a new motherboard?

Thanks!