Please clarify. The system shuts off and does not restart correct? Are you able to turn it back on immediately or do you need to wait sometimes?
Shutdown tells you you're triggering an emergency either due to temperatures, or power delivery. You've monitored the temps and it sounds like they stayed low, so we'll rule out heat.
Power. you've replaced the PSU. Next, replace the power cable, if you didn't previously. Test another outlet if you can. If that all fails, you're back to the VRM on the board being your final culprit. In that event, replace or RMA the board.
Sounds like you've ruled everything out that you can. Let us know if you have any other questions after you've swapped the board.
Get Hardware info app, run it and watch all the temps, the board VRM, CPU, GPU. see if something is running hot. Also, you can see the power rails on this app for PSU.
Check the code light that comes on when the shutdown happens
Check the Windows system logs for hardware errors
Try running with the case open in a cold room to determine if its temps. If it runs longer its temps on something and hardware info will show it out.
Make certain all hardware is seated properly
Try unplugging extra drives and only using boot drive, a drive may be getting hot, especially if you have SSDs in really confined spots like on the back of the motherboard in the right side of the case
then take it in
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