Few things to check:

• make sure all wires are secure and snuggly attached to everything. Including the rest/power button switches, PCIe devices, sata devices, etc.

• double check temps before crash to see if theres any issues while idle or under load there.

• check and possibly post your windows logs and event viewer logs to look for possible issues leading up to right before the power fail events.

• do you have any overclocks applied? (Including xmp/amp) if so set your motherboard back to defaults and test again.

• make sure BIOS firmware is up to date (might as well make sure all your other drivers are up to date as well).

• try each ram dimm one at a time in both primary slots, this may help determine a dimm issue.

Borrow or using another power supply would be the best way to full confirm or deny the issue being a power supply failure. When a computer just instantly powers off like that its usually either a power failure or some kind of critical harware failure hitting a failsafe and shutting down. Normally this issue points to motherboard or psu issues in my experience.