Intel Motherboards : Suddenly not booting with all cores enabled

My setup was working flawlessly for a week and one day few minutes after boot Windows suddenly froze for no obvious reason and after hard reset no system boots with more then 3 cores enabled, and even with 3 sometimes doesn't boot. CPU temperatures are never higher than 30-40°C. I checked everything inside case, everything seems correctly installed. Intel Processor Diagnostic Tool passes with no errors (also with 3 cores enabled). I tried memory in different slot, didn't help. Nothing is and never was overclocked (as far as I know, I use default UEFI settings), voltages are on standard levels. I have latest UEFI.

 

I'm using following components:

Intel i5-6600K

ASRock Z170 EXTREME4

HyperX DDR4 8GB 2133MHz Cl14 Fury Black [HX421C14FB/8] (single stick)

Zalman ZM600-GSII 600W

CoolerMaster Hyper TX3 EVO

 

I use integrated HD 530 GPU.

 

If it is of any help, I have dualboot with Windows 10 and Opensuse Tumbleweed. POST is fine, GRUB works fine, with all cores enabled Windows hangs at blue logo (with no circling dots) and GNU/Linux after which looks like NMI watchdog initialisation, the last line trace looks like "[0.559] #0 #1 #2[", which I imagine could probably indicate, that something is wrong with core #4, or when system tries to initialise / use last core.

 

I did IPDT for UEFI, since I cannot boot into any system with all 4 cores enabled. The only thing out of order seems to be this, it's same with 3 and 4 cores enabled:

 

When I run test on Windows with 3 cores enabled, detected CPU frequency is 3.4996 GHz and clock frequency is 99.9912 MHz.