How to defeat the spontaneous turning on of the PC immediately after shutting down?

Question

Iron

Problem: The
PC randomly turns on after a graceful shutdown of the system, about 4-6 seconds after shutting down.
The power indicator goes out, after 4-6 seconds it lights up again and the PC starts correctly.
The time interval until automatic switching on is the same, they did not time it with a chronometer, but according to the mobile stopwatch 4-6 seconds.
If you send Windows to "Sleep", the same story - after saving the RAM dump and turning off the power, the PC starts after the same 4-6 seconds, and Windows performs "Resume from Sleep".
If you catch the moment and in these 4-6 seconds until it cuts back, turn off the network cable from the outlet, then when you turn on the power back, it DOES NOT START. Those. does not work out exactly turning off the power, but does not start immediately when plugged into a power outlet. It is important.
PC configuration:

• Motherboard Gigabyte GA-PH67-DS3-B3 (rev.1.0) with Intel Core i5 processor (Sandy Bridge)
  
• BIOS latest version (F5)
  
• PSU 400 W known good
  
• From the rest of the hardware in the computer, 8 GB of RAM, a PCI-E Nvidia GT 710 video card is known to be good (passive cooling, no additional power), two 7200rpm hard drives - and that's it, there is nothing else.
  

What I've done so far (didn't work):

• Replacing the power supply with a known good one is the same garbage with a completely new power supply,
  
• Reinstalling Windows - to no avail,
  
• Disabling the body buttons - both Power and Reset in general (removed all the pins from the Front Panel) - starts by itself, it's 100% not in the button
  
• Resetting the BIOS from the BIOS itself
  
• Clear CMOS jumper reset
  
• Careful study of all power settings in the BIOS, including non-obvious ones. Wake-On-LAN is disabled. Power On Events are all disabled (no power on keyboard, power on ring, power on mouse, etc). Power on after PWR failure is set to SOFT-OFF.
  

Interestingly, there was no such problem on this motherboard before. It is now impossible to find the same PSU and the same case with which she worked without this problem.
With 3 other PSUs, we observe the described problem.
If you hold the Power button for 4 seconds and turn off the computer forcibly, then after that the problem is not observed, it will not start itself.
Any options like ErP tried to turn on and off - no effect.

Answer 1

Change the battery on the mother. Or just take it out for a test.

Answer 2

1 BIOS firmware, cartoon firmware, cartoon replacement.
2 Ground loop/layer pierced?! Lay out on the table without a case and turn it off from Windows. (Screw to the body through the dielectrics if that's the case).

Answer 3

Gigabyte motherboards on a number of chipsets suffer from this. Sometimes it is treated by replacing the battery. Sometimes by flashing the bios. It just went away for me after a year. True, at one time I almost stopped turning off the PC, maybe the constant load brought me to my senses. If it really infuriates, then change the mother to a new one