[edit: added video link and pics]

Gigabyte z77-d3h stuck at boot screen - Why? - YouTube

This issue has gone on for at least 2 years without a permanent workaround. I initially thought it was a BIOS problem, which was why I switched from the default Gigabyte BIOS to the F23B tweaked BIOS provided here.

Unfortunately, what happens is that, at times, be it when resuming from sleep or booting from scratch, it reverts to the OLD BIOS screen, gets stuck there forever and does not respond to keys. Even after removing CMOS battery, unplugging power cord, it gets stuck at OLD Bios too.

The only time it comes alive is the occasional booting up (without any other troubleshooting steps) and it detects "boot failure" and allows me to boot normally into Windows.

Any advice on what could be the issue? I have replaced every conceivable part, ranging from RAM, to hard drive, to GPU and PSU. The only things I have not swopped out are the CPU and Mobo itself. Could it really be the mobo is "dead", despite being able to boot up properly from time to time? I really don't want to change cos it's different to find a NEW and compatible board that pairs with my old but gold 2500k these days.

Hope to get some advice here. Specifications below:

Intel i5-2500k 3.3Ghz @ 4.3Ghz (1.3v) & CoolerMaster V6 Air Cooler

Gigabyte Z77-D3H (Rev 1.0, F23B Tweaked BIOS from TweakTown) Motherboard

4 x 4Gb Team Elite DDR1600 in Dual Channel

Gigabyte GTX750Ti 2Gb (Monitor running @ 1440p resolution)

2 x 512Gb Samsung 850 Evo SSDs (1 for OS/Programs, 1 for Footage)