ASRock X99 Extreme6 Unable to Power On Sometimes

Hello,

Shortly: After pressing "power on" nothing happens. Suspect software/UEFI bug.

Context: I bought computer 2 years ago with the following configuration:
CPU - Intel Core i7-5930k
Matherboard - ASRock X99 Extreme6
RAM - Crucial Ballistix Sport < BLS4C8G4D240FSA > DDR-4 DIMM 32Gb KIT 4*8Gb
GPU - MSI V317 GTX 980 GAMING 4G
PSU - Seasonic 1250W (SS-1250XM) X-1250 Gold Modular
SSD - SSD Samsung 850 Pro 512GB (MZ-7KE512BW)
UPS - APC Back-UPS ES 700VA (BE700G-RS)
Tower - Thermaltake Core V71 Black (CA-1B6-00F1WN-00)
All that stuff worked as expected: no issues, no bugs, no BIOS/UEFI updates (because all works).
2 years of work - so I assume I had assembled it correctly, hadn't it?

Now it is very hard to power on computer (PSU turn on/off dance, clear cmos, etc).
Here is successful scenario:
Computer is off
I press power button
I see lan led is on
I see onboard power button led and reset button led are on (red light)
I see Bios_A red led
I see dr. Debug working
PSU is on, lights on tower working
Computer boots Successfully
Using my OS (Debian 8 or Windows, doesn't matter) I shutdown computer
It stops all that stuff
Shutdowns HDDs, SDDs
Turns off PSU
Turns off onboard power button led and reset button led
Turns off Bios_A led (there are two BIOSes)
Done
Now the failed scenario:
Computer is off (after successful shutdown)
I press power button
Nothing happens, no lights, no button led, no psu power on, no fans. All looks dead.
But I see that LAN led working. That is.
Pressing power button changes nothing
Holding power button for 4-6 sec turns off LAN led. Pressing power button again - send us to step 3
Trying to turn off PSU, wait 15-20 sec, clear cmos - sometimes it starts (1 of 10)
Sometimes when I turn off PSU in that state it tries to turn on and immediately switches off.
That "turn off/reset/turn on" dance after 10 or more tries finally turns on my computer.
Then there are 2 cases.
First one: computer turns on for few seconds, loads UEFI, pass post and then turns off (we now again in failed scenario)
Second one: it turns on and work as expected. Any number of reboots - all ok. System is stable, no BSOD, no kernel panics, etc. Only shutdown cause "failed scenario".

One last scenario - unfinished shutdown:
Computer up and running
I shutdown it or I go to UEFI, change any setting and press save (looks like this method uses something different instead of reboot)
Fans turned off
System halted
Onboard Power Button Led stays ON
Onboard Reset Button Led stays ON
Onboard BIOS_A Led stays ON
LAN Led stays ON
Holding power button for 4-10 seconds doesn't turn off LAN led
Now I'm in "failed scenario" again
Dr. Debug doesn't help (few times I saw code 79, but usually it turned off). I cleared CMOS many times.

What I did:
Did a memory test - all ok
Disconnected video card and other devices, tried start only motherboard+cpu+memory
Tested PSU by connection PINS 4+5 (manual power on)
Tried to switch to BIOS_B
Tried to update bios to the latest
Replaced battery (old one have about 3.0V, new one was 3.18V)
Checked my assembly again (keeping in mind that it have worked for 2 years)
I also can check PSU output voltage, but I'm sure PSU is ok.
I don't know what to check next, how to debug boot process.

Any ideas, help are appreciated.


thanks
alexsunny