Question PC randomly shuts down (not PSU problem)

[Maj_Kusanagi]

Hope if anyone can provide insight: My PC started to random shutdown a few months ago (everything suddenly turns off, event viewer only shows kernel power error). It happened first in June, so I reseated the RAM and it didn't happen again until 1 month later in July. Then it became more frequent from August up to now. But the random shutdowns would happen very randomly (could happen few hours after turning PC on, after 1 day usage or even 3-4 days) , sometimes while just idle, browsing, gaming, or leaving the PC on overnight. Besides the random shutdowns, there were a few instances where the screen froze during gaming or light usage (keyboard unresponsive, fans still spinning normally) requiring a hard shutdown, with Windows only showing the kernal power ID 41 after turning it back on. Sometimes after the shutdowns, the PC does not boot up properly (case & mobo light turns on then off without POST, or powering off right after POST beep, HDD light not active yet), requiring me to push the power button a few times before a successful boot into Windows. After switching to integrated graphics, there were a handful of times where I was in desktop or watching youtube, the screen turned into a solid color (yellow, red, or green) with system freezing requiring a hard reset.

Steps taken with no fix:
-cleaned dust and reseat different RAM slots
-no errors on Intel processor diagnostic
-memtest show no errors
-temps show all normal
-tested SSD on Crucial Storage executive
-reverted BIOS settings
-removed graphics card using only integrated graphics
-swapped PSU with a friend's working one

So last week I finally took the PC to a repair shop, where the guy tested my PSU with a device (no abnormalities), tested my ram again without issues, ran OCCT power test and the PC would lockup within minutes. He swapped my CPU with his i5-6400 and ran the same test for 18 hours with no problems. The next day he sound convinced that my i5-6600 is the culprit, explaining that the CPU and integrated GPU are stressed at the same time. However I'm not 100% convinced and think it may be a motherboard problem as well. After taking my PC back home, I repeated the same OCCT power test and noticed my CPU would reach 100c and freeze within minutes (requiring hard shutdown). I ran the other OCCT stability and linpack tests for 1+ hours with no problems (temps reach around 70c), so it's only the OCCT power test that would cause such high temps and freezing. At this point I'm not too sure if it's the power test causing such error or really a faulty CPU. Everything runs fine and smoothly when my PC is on, and I am unsure whether to buy a used CPU or motherboard next before wasting more money.

System specs:
Intel i5-6600 (never overclocked), Gigabyte H170M-D3H, Antec NeoEco II 650w PSU, 8GB Adata XPG DDR4-2133 RAM, Crucial 250GB SSD, 1TB HDD, Windows 10 Home 1909, standard case and peripherals

 

[Lutfij]

You can't overclock a non K suffix processor, let alone on an H series chipset for the Skylake and above platforms. You're supposed to be on version 2004 for Windows 10. Which BIOS version are you on at the moment? How old is the PSU?

What was the make and model of your friends PSU? What GPU are you working with?

 

[Maj_Kusanagi]

You can't overclock a non K suffix processor, let alone on an H series chipset for the Skylake and above platforms. You're supposed to be on version 2004 for Windows 10. Which BIOS version are you on at the moment? How old is the PSU?

What was the make and model of your friends PSU? What GPU are you working with?

Haven't updated to Win 10 version 2004 yet, never had any operating system problems before. Likely not a Windows issue as the system lockup happened once when I left PC idle in BIOS.
BIOS version is F23a , my Antec PSU only around 2 years old. The PSU I borrowed is a Cooler Master 450w, total power usage quite low on integrated graphics. My GPU is a RX570 4GB, I have removed it from my system over a month now, using only integrated graphics. Random shutdowns occur using both PSU's, while voltages in bios show normal.

 

[Maj_Kusanagi]

Just an update, ran Intel Processor Diagnostic Tool again choosing burn in (2 hour long test). After it reached CPU Load test, screen turned black (no signal) and system froze with keyboard lights unresponsive, but the heatsink fan kept on running high speed as if still doing the test. I let it stay in this state for 15 mins, nothing changed so I hard powered off. Can I be certain this incident is proof of a faulty CPU?