In a previous life, my friend Max was always sending cases of power supplies back to the wholesalers.  It's super easy to reduce production costs by cheaping out.

We would skin the first power supply in the case, look to see if anything obvious was missing, and set it up in one of his chassis for a load test. We regularly had X watt PSUs fail to deliver anything like X. One was so bad that when we started the load test, the line on the oscilloscope went from 12v to ... zero.

If the first one passed, we'd skin the rest (we never used the crap cases), see if they were all the same, and then add crowbars and put them into his chassis, which was designed for good airflow and signal damping with a naked PSU.

And at least once, they changed half-way through the case!

--dave

From: Evan Leibovitch via Talk <talk@lists.gtalug.org>

Unexpectedly and suddenly,  my not-so-old daily driver PC has just given up.
It boots but either freezes or reboots after between one and three minutes
of activity.

It looks like you have tried all the obvious things.

First thoughts:

Before everything else: Make sure you have a backup.  That may involve 
removing the disks and hooking each to another computer.

How can you run memtest in less than three minutes?

It sounds like an overheating problem.  You've tried to address that.  
Grasping at straws: are there any fans on components that are not 
spinning?  The processor says that it is happy.  You've removed the video 
card.  Are there any other fans just for one componenet?

If you go to the firmware ("BIOS") settup page on boot, and you let the 
machine sit for three minutes, does the machine reboot or freeze?

Have you tried reseting the "BIOS" to default or factory settings?

Years ago, I was surprised how often a power supply misbehaved.  So I keep 
a spare in stock.  But I haven't needed one for at least 15 years.

Good luck.
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-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
davecb@spamcop.net           |              -- Mark Twain