Hello fellow members, this issue is already resolved (sorta) but I'm posting here just for reference in case someone else comes up with it, also I'm not sure if its MB related or GFX card related.

Specs:
i7 5930K (stock clock)
Kingston HyperX 32GB KHX2400C12D4/8GX (SPD 2133)
Rampage V Extreme bios 1701
Zotac GTX980ti AMP (3) - (BIOS Version 84.00.36.00.2F, Driver 353.82)
Samsung 850 Pro 512Gb (2) Raid 0
WD Black 2TB (2) Raid 1

Issue: system cold boots and shuts down immediately, every time, no debug code, no beeps, it just shuts down like if the PSU tripped, restart doesn't experience this problem, only when cold booting from a complete shutdown, get around it by holding down the power button for 4-5s and press power again, then it boots up fine, able to replicate multiple times.

Solution: I have unplugged everything unrelated and not necessary so long as the system boots up and found the culprit, 3x monitors plugged into the same GPU1 (top most GPU closest to the CPU) using display port connectors :eek: .The monitors and connections are: Asus PG278Q (DP), Asus VG248QE (x2 DP and DVI), Asus PB279 (DP). I have switched to 1x HDMI, 2x DP and 1x DVI to connect the same monitors and problem goes away. So apparently this Zotac 980ti AMP or just 980ti in general (?) doesn't like having 3x monitors connected to it via DP?

I also tried switching to 3x VG248QE surround and PB279 accessory, using DP on GPU 1, 2 and 3 and DP for the accessory display on GPU1, no problems what so ever.

I have already contact Asus Tech Support regarding the issue, it appears it's a rare case..