use torch to check the output caps in the PS. look for the poped or leaked ones.
if no faults are found. try this:
power off the PS from wall overnight (8 hrs+). then see if the problem still exist.
let us know you findings.
unlpug ps from wall. wait 1 min,
unplug the all SSD and HDD both sata and power.
take out the display card (if you have one)
connect to monitor using on-board (hdmi or dvi or vga)
power on pc and use mem86+ to check ram.
after testing, if all ram sticks are good turn off pc.
turn on pc,
try a few times, does it auto power down?
just some info,
my guess is the PS is faulty, i believed you will see the bad caps after you take the PS out for a closer view. but still do the test to make sure. this is also a good time to clean out the dust.
PS. this is like my samsung tv, it will reboot once, then everything is fine, two weeks later, it will reboot more than 20 times, before i can watch tv.
i replaced 4 caps and the problem goes away.