Question Only one core is visible in Linux - Ryzen 5 2600 (if you can boot at all)

[marcus0]

Hello guys and gals!

I'm brand new to the forum. I haven't registered before, because I was able to solve my previous issues by searching existing threads.
Until now.

Symptoms:
-Can't boot the computer with Windows, even from bootable USB that works with other computer: it freezes about 3 seconds after the boot process starts and the keyboard powers off (num lock doesn't work, keyboard illumination is gone)
-Only way that I can boot is with Linux, and only by adding the kernel parameter spec_store_bypass_disable=off, and even then I can only see one CPU core via hardinfo, lscpu and cat /proc/cpuinfo with a Ryzen 5 2600
If I don't pass the parameter than booting stops either with a "do_IRQ: 1.55 No irq handler for vector" or a kernel panic at _raw_spin_lock, tried with several different USB bootable recovery tools.
-When I start memtest and set it to round robin between CPU cores, it freezes (!)
My goals:
I don't know whether the CPU or the motherboard is the culprit. I don't want to buy both, but I'm fairly certain one of those is causing the problem. Could you help me find out?

Further context:
-RAM and CPU was overclocked via UEFI with more than adequate cooling for about 2 years. When the PC was first powered on after a day that it had no power (switch on the power socket was turned off) it had minor troubles starting: it powered off a couple seconds after I turned it on, then automatically turned on back again, but I've checked the UEFI settings and the overclocking weren't set back to default. This was fairly regular in the past.
-I've used Windows previously, but one day it just failed to boot. Recovery didn't boot either, but it did show the repair screen at boot. It just froze during the early stage of booting. Power off was the only thing that helped. That's when I've tried to boot from USB the first time.
-Had to update BIOS to version 1.10 to 4.60 (https://www.asrock.com/mb/AMD/B450M Pro4/index.asp#BIOS) just to get this far: before that the gparted USB stick froze at the boot menu (normally it counts down from 30, it stopped at 30, keyboard & num lock not responding, no keyboard illumination/power)
-Tried different keyboard, didn't help
-Reseated & repasted CPU, reseated disks, GPU
EDIT: I've done a full memtest with the default multi-core settings (just F2 after it starts) and it passed.

Hardware info:
Ryzen 5 2600
Asrock B450M Pro4
2x8 GB RAM
USB keyboard, backlit

 

[rgd1101]

check cpu temp?
make and model of the psu?

 

[marcus0]

Thank you for your lightning-fast reply!
PSU: BitFenix Formula Gold 550W
Other components (for wattage calculation):
1xHDD (Seagate Barracuda 2TB SATA3)
2xM.2 SSD
Nvidia GTX 1080
CPU temperature: 42-49 Celsius, didn't get higher with stress --cpu 8

 

[marcus0]

I've reseated the RAM, and now memtest freezes right on the first test. Tried it with different settings - even with the one that worked before-, and it always freezes, keyboard is not responding (num lock doesn't work, (c)onfiguration button does nothing). Tried it with different physical configurations, i.e. only one memory stick, switched the slots, nothing helped so far.
RAM: 2x8GB 3000 MHz G.Skill Aegis kit was OC'ed from the base DDR4-2133 1.2 V to DDR4-3000 1.35V

 

[rgd1101]

check for bend pin on the cpu?

 

[marcus0]

Dropped it off at a PC repair shop.
Thank you for taking the time to reply!

 

[marcus0]

Replaced the motherboard and the CPU and now it works :O
Thank you for your time!