kiwifidget:

 

Everything already up to date, and all tests passed.

 

I noticed an audible fan noise during the cpu test.

 

Pretty sure I've heard that same noise about the same time the clock stops.

 

I've heard cooling fans spin up when there is a problem a number of times.  I figure its some kind of fail-safe thing.

Is it practical to erase the machine and set them up with a fresh Windows installation?  If it works fine, you've eliminated a hardware fault.  If the same thing happens, it's most likely a hardware fault.  With the machine being under three years old you could take it back and ask for it to be repaired, but the first thing they will likely want to do is factory reset the software.

One thing to try while I think of it.....  do CHKDSK (did you do this earlier?) and then an SFC / SCANNOW

Use SFC Scannow to Repair Windows System Files (lifewire.com) 

kiwifidget: Even the keyboard and mouse?

Except those. Remove everything else like flash drives, plug-in wifi adaptor, external webcam, USB printers, etc. That will eliminate a failing peripheral, which is a common problem.

chkdsk came back clear

sfc /scannow found some corrupt files and repaired them

but the problem remains.

Doing a clean install on a new disk and seeing what happens could well be the plan.

kiwifidget:

 

No AMD graphics, it's an Intel PC.

 

I have an Intel PC with AMD graphics installed by Intel itself.

Clean install is the way to go.

kiwifidget:

 

chkdsk came back clear

 

sfc /scannow found some corrupt files and repaired them

 

but the problem remains.

 

Doing a clean install on a new disk and seeing what happens could well be the plan.

 

 

 

Sounds like a faulty SSD. Or could even be the motherboard. Something is causing it to hang. Some of those AIO are not very reliable.