I had simmilar issue and found out that
a) my 3.3v was low (like "*(removed by Cool GTX) it is 3.0v!") and
b) my 12v was low under cpu load (like went down from 12.3 to 11.8), while under gpu load it was fine.
c) this problem was getting worse and worse, like from shutdown once a week to couple shutdowns per day.
I tried to work with 24pin contacts (cleaning, pushing to get better contacts), that gave me a minor improvement. And so I ended up with ordering new cables (still waiting) and removing all cables from my system, checking them (all looks fine), checking connectors on both psu and mother board (fine too) and reconnecting it all without cable management (because I still wating new red-and-black power cords :) ).
Routing cables without any hard bends (I had especially tight circle bend near psu and I believe that this elastic tension gave me bad contact on my 24pin from psu side) gave me great improvement: now my system doesn't shut down at all again, 12v drops down under cpu load to 12.2, but 3.3v still a bit low, like 3.28v.
But there is more: suddenly I discovered that my USB type c on front pannel didn't work and even more: connecting something to it made my bios turn off PC due to USB over current after POST. It turned out that you actually could connect that ugly USB 3.1 gen2 wire upside down and that would short-circuit something to the ground. And that is awesome: despite that these header and cable have a key that should prevent it from connecting upside down - that actually don't work and you can do it easy :)
After I fixed this problem my 3.3v had went up to normal 3.32v, but overal system stability was achieved before with good contact on 24pin motherboard cable.
I can guess that these shutdowns was motherboard "fault" or some anti-surge protection, because:
a) I tried to keep my system shutdown if power was lost (there is option in bios called like "Restore on AC Power Loss") and also I know that any psu protection just shut it down, but not power cycle your system;
b) I tried to disable any OS restarts on any faults - still power cycled;
c) I actually repeated that issue with touching and "shaking" 24pin connector and that had gave me power cycle even if my motherboard was instructed to power off after AC power loss.
But sad thing that there was no messages after post while I hoped to saw something like this:
PS this maybe ugly english grammar of mine is result of reading "english grammar for dummies", so I am kind of trying to use something exept present and past simple :) Please be forgiven, it will be better I hope :)
edited your post Cool GTX ... language removed
post edited by Cool GTX - 2020/06/05 18:57:10