One other (rare) event that I forgot to mention. Sometimes a fan gets stuck due to reasons explained later if relevant. Once the fan starts spinning, it keeps spinning. But you should be able to stop the fan. And it restarts every time and quickly when released. A fan that only slowly returned to full speed may not always start - especially when cold.What is strange is that during both today and yesterdays tests the PC didn't once shutdown, and that's makes this issues so frustrating
If the fan does not spin and if that unique power supply design contains a temperature monitor (not required but some have that feature), then a too warm supply will declare a not 'Power Good' and shutdown.
That would explain your ‘I cannot get it to fail’ scenario.