This has been running fine for about 3 months. All updates are routinely applied, the only added hardware is a fast USB3 Sony 64BG flash drive used for extra storage that is left plugged in. The machine only gets used intermittantly for software testing.
As of yesterday it is no longer usable for any purpose. It will boot and once it gets to the lock screen it shuts itself down within seconds. If I'm quick I can get to the logon screen and enter a few characters of the password, really quick I might even get to the desktop. Then the display goes dark and after 30 seconds or so it powers off.
The only change made yesterday was to reformat the USB flash drive from exFAT to NTFS. However even removing this drive and rebooting changes nothing and the machine worked for 2 months without this drive. The drive had nothing on it aside from scratch files, and little of that even before reformatting. Perhaps just coincidence?
I have done several full resets and reinstalls by booting with Alt-F10. When I get to the end of that process it runs fine... up until I have to go through a restart. This usually occurs because I have uninstalled some crapware, or ignoring that when I change the WiFi network connection from Public to Private or anything else requiring a restart.
I believe I've ruled out a bad power supply, since it will run for hours as long as I never restart or shut down.