My old rig is: P8Z77-V, Intel 3770k, Kingston DDR3 1600 8G, NVidia GTX 1060 6G, Noctua NH-D15, Antec 650W PSU, Win10 latest.

This is really weird, I've never seen my PC RANDOMLY shutdown itself before, and when it's shutdown (the LEDs on motherboard is still on, so it's still powered), I can no longer power it up by pressing the power button. The only way to boot it up is to switch off PSU, wait for the motherboard to completely turned off (takes several seconds for the lights out), and then switch PSU back on, press the power-up button, and then my PC is able to boot normally.

What I can confirm before:
No OC(stock freq 3.5~3.7), iGPU: Fine
No OC(stock freq 3.5~3.7), GTX 1060: Fine
OC to 4.5(1.25V stable, fullload 65C), iGPU: Fine

The problem did not occur until last week I install my GTX 1060 on it with previous stable OC 4.5 (1.25V), then it starts to randomly shutdown. By saying random I mean it happens no matter how much CPU load is, it happens when I play games, happens when idle, yet nothing special in the system events viewer, it just says the system restarted unexpectedly. The temp are below 70 when fully stressed. The real weird thing is that I tested my PC with Prime95 for hours and it's quite stable, and it can last as long as 12 hours to eventually shutdown.

At first I guess, Ok, maybe I exact a little bit too much, so I lower the freq step by step until 4.1, which is by no mean unreachable for 3770k, and it still randomly shutdown. So, I start to think if it's not entirely the OC freq issue.

Now I set it back to default to see if it can last for a day or more, to make sure the motherboard and PSU are not damaged, and then further raise the freq up gradually, to see exactly where the problem is, it should take quite a few weeks I think.

Please anyone has ever seen this phenomena before (randomly shutdown) and whatever possible reason it might be? Thanks.

----------------------------------------2018.2.24. Update
switch back to stock default BIOS settings, it still random shutdown, but this time when I boot it up, it clearly says "power supply surges detected during the previous power on. ASUS Anti-Surge was triggered to protect system from unstable power supply unit!"

Well, since it's at stock settings, this definitely never happened before, default autop turbo will only set to 3.7 and fullload temp is about 50, I think it's not about OC freq, something is wrong with power related units, either PSU or motherboard, I'll do further test.:(