Your problem with the Seasonic was probably aging capacitors Max.

Lifetime of electolytics is an inverse function of temperature and some specific makes were very poor.

A PC PSU is basically always ticking over when powered up waiting for the trigger from the on off button.

When the button is pressed there’s an instantaneous check for current drain which if deemed excessive will prevent the DC voltages establishing. If a bad capacitor is present things will go awry with normal current deemed too high probably because proper voltages are not being established.

If you open the bad PSU case you would most likely see one or more capacitors with a bulging top.

The same problem capacitors plagued motherboards ten years ago.

Allan G3PIY

Agree power supply can caused weird faults. I had one where the PC could not be turned on with the front panel switch, but would turn on if the main AC outlet switch was cycled. Friend had same PSU (Seasonic) they no longer needed, but was identical model and age as we had got a deal by company to build our two PCs at same time to same spec, so he sent that to me. Tried that. Exactly same fault, so assumed PSU not at fault but then PC decided not to turn on at all. Changed motherboard, same fault again. As a last resort I bought a brand new PSU (Corsair). Turned on, all sorted. Incredibly, friend's PSU they had lent me must have had exactly the same fault on it. What's the chance of that?!

Had the new Corsair PSU now for maybe three years. No issues whatsoever. Possibly illogical, but will probably avoid Seasonic PSU again in future!

Max