[SOLVED] 8-cores FX-8350 stock + Prime95 = freeze, but okay if cores 5 & 6 are disabled ?

[Teran1982]

Current system:
Cpu AMD FX8350 on 6 cores (5&6 core disabled in BIOS) @4.3GHz
Ram Gskill Ripjaws 1600xmp 24GB (2+1 modules)
MB Sabertooth fx990 r2 asus
Psu CorsairRM750x
GPU ASUS R290 STRIX
Ssd Samsung evo 250 + 4 HDDs

Watercooling setup and working on CPU
temps CPU 35c GPU 55c (under load)
CPU 26c GPU 35c (idle)

History:
I started noticing problems 2-3 years ago. PC was freezing randomly even when idle. Lost a memory stick. The problem continued with random shutdowns. I found that the problem for the shutdowns was the PSU and I changed it with a new one. But the freezes continued. I finally found a clue, when I run Prime95 on all cores, PC freezes (all ok on screen but no input possible) almost instantly. Then I entered the BIOS of my MB and found that problem only goes away if i disable cores 5&6.Then I can run the program all day long without any problem. I am at a loss, what could be wrong? Is it a CPU problem or MB, due to that I had a spoiled module already?
How do I continue troubleshooting now? Can I check somehow my hardware further? Tried clear-CMOS, stock speeds on all components, underclocking all cores, different set of RAMs, Format, BIOS update, extra cooling etc... No solution. Unfortunately I don't have another compatible PC to interchange MB and CPU to see who is messing me up from the two! Any clues guys??

EDIT: Is the memory controller on FX included on every set of cores independently? My thought was bad memory controller, could it be?

 

[kerberos_20]

memory controller is independent from cores, but overclocking through FSB can affect mem controller aswell

as for those freezes on all cores, that sounds like not enough power comming from VRM, try increase LLC or rise cpu voltage to 1.4v (for test)
4.3Ghz shouldnt really matter, those cpus can handle up to 5GHz on 1.55v i think

 

[Teran1982]

Thank you, I'll try later this year and if i have a success i will post again!