x570 taichi random power off issue

Hi everyone,

I can't figure out to edit the other message so I'm just going to dive in.

I have a four year old x570 Taichi machine which is randomly powering off instantly, as if it's lost power.


Specs
5950x, nvidia 3070, 128GB from the QVL list (using XMP), five Sata drives, no nvme, bios 5.60, meshify 2XL case, noctua D15 with two fans, windows 10. Initially an HX1000 psu.

Attempted fixes:
Ive replaced the psu with an hx1500i. Still happens.
Removed Dimms, swapped around, run on one. Still happens.
Changed graphics cards. Still happens.
Repasted and re-installed CPU. Still happens.
Removed unnecessary cards/usb. Still happens.
Changed keyboard. Still happens.
Memory down to 2666, no XMP. Still happens.

Have not tried a different CPU, motherboard or mouse.

Bios:
No overclocking, everything on auto kind of thing, memory using XMP profile (slowing doesn't help).

Temperatures
CPU will idle <50 in windows. Low numbers.
MB will idle <40 in windows.
SB will idle around 50 in windows (see below....).
The machine will die at idle and under no load.

When it crashes:
It can shutdown in windows 10 at idle.
It can shutdown in bios.

Strange evidence:
Sometimes I would heard the SB fan spin up before shut downs. SB fan is semi passive in bios.
Running hwinfo logging at 100ms polling shows the SB temperature will read -100.3C (as in minus one hundred celcius) for 200ms immediately before the shutdown. You might also see 100ms of SB fan just before it all turns off.
Sometimes the SB temperature sensor isn't there at all in hwinfo, sometimes it appears mid session. Will be missing in hwmonitor, too.
Very occasionally you will hear the SB blip on/off for "a second", and when it does this you will see blips of -100C on the SB temperature sensor but it won't crash.
Separately it does look like there's a very subtle but mildly dark area on the 5950x on the memory facing side, about the size of a small finger nail. Really realllly subtle though.


Does this sound like the sensor is misbehaving?
Can I disable the sensor?
I tired to find it on the motherboard but failed, can anyone say where it is?
-100C is crazy talk, is that actually a huge positive value going out of range and looping around?

Thanks everyone.

K