Computer won't POST/boot

I would actually disagree with you. In general, I think Silverstone is one of the more reputable power supply manufacturers. They aren't recommended much becasue they're all expensive. But from what I've seen, Silverstone PSUs are good.

I think I had an older one that either started dying or had an incomparability with my system.

This caused so much trouble because a partly-functioning PSU is very difficult to diagnose.

 

We saw the 5VSB getting shorted to the 3.3 V. And that intermittent short appears to be inside the PSU. The actual problem was obvious once numbers existed. The suspect was immediately reduced to only two.

Did you happen to shake and hear a noise inside the PSU?

Have you since measured voltages to confirm this new PSU is functional? Normal is for a defective and new PSU to still boot a computer. Measuring is how to identify a defect that would otherwise not cause a failure until after its warranty expires.

 

never cheap on the psu

 

From now on the first thing I check when the PC is shutting down for what seems like no reason (randomly) is the power switch. I swap it for a known good switch I have on hand.

 

WINNER! First to hit the nail on the head!

And, I am proud of Westom, as, he helped and did not preach.

Thank Goodness, glad you got it fixed...
You have now have enough testing experience to put it on a resume.
Happy 'Puting.