On Mon, 11 Aug 2014 03:52:30 -0500, Paul wrote:

With your symptoms set, I would tend to think power supply
at this point.

One thing I would check, is the green LED on the motherboard.
Asus motherboards usually have a green LED. The idea is,
when working on the computer, you don't pull DIMMs or cards
out of their slots, until the LED is off. The green LED
is tied to +5VSB from the power supply, and so it's an
easy way to monitor that voltage rail.

When your power supply shuts off, I would want to view
the green LED at the same time. It is wired to +5VSB, and
that voltage is part of the PS_ON# circuit as well. The
PS_ON# signal gets released, if for any reason +5VSB ever
drops out.

The power supply consists of two pieces. The motherboard
turns on the main section, using PS_ON# signal on the main
power supply cable. But the PS_ON# driver is powered by +5VSB,
and the supply shuts off, if the always-running +5VSB goes away.

+-------------------------------------------+
(PS_ON#) | |
v |
AC --- HVDC ---+--- Main_Supply --- LVDC 3.3V, 5V, 12V, -12V |
| |
+--- Standby_Supply --- +5VSB ---- RAM in standby |
---- WOL for NIC |
---- USB waking |
---- Helps with PS_ON# --+
---- (Green LED)

The +5VSB has a 2 to 3 amp rating (check PSU label for details).
Shorting +5VSB to ground, causes the chip on the motherboard
driving PS_ON# to get switched off, so then the Main_Supply goes
off. The green LED on the motherboard should not wink or
glitch - the switch on the back of the power supply, is
the only thing that should extinguish the Asus green LED.

Not all motherboards have that monitor LED. I think
every Asus motherboard I've owned, had that LED. And
it's great for observing the voltage that can lead
to the supply switching off.

A more likely cause, is the PSU is overheating, and
while the PSU has a current limiter, it's possible
it is thermal (heatsink inside PSU overheats, supply
goes off). If for any reason, the thermal sensor inside
the supply isn't working right, that could shut it off.

And your Asus motherboard will restart the system after
a +5VSB failure, if the BIOS is set that way. My systems
here, I set them to not Autostart. It could be that
yours is set to run the computer after a power failure,
and that's why it boots after the power failure. You
can adjust the BIOS, to have it sit in the OFF state,
if such a failure occurs. When you build up server
systems, you usually want it set to Autostart so all
your servers recover without you being present. For
desktops, it's the opposite.

Paul

Paul, thanks for your highly detailed reply. After you and Bob
suggested overheating as a good possibility, I did some investigation
and have interesting (and embarrassing) results to report. My case is
an Antec P280, which has dust filters on the front intake fans, and on
the power supply fan. The power supply arrangement in this case is on
the bottom of the case, with the intake facing down, and the filter is
between the power supply intake and the bottom of the case (which has
a perforated section to let air into the power supply).

I checked both filters and found them clean. I blew out all the fans
with compressed air and found almost no dust on them. The only thing
I couldn't easily examine was the power supply fan because of the
location of the power supply. So I turned the case upside down so I
could verify the power supply fan was running. I was astounded to
find the perforated case intake area for the power supply almost
completely clogged with dust!

What was happening was that the dust was accumulating on the
perforations and not even making it to the filter. Since I frequently
checked the filters and found very little dust, I assumed everything
was OK.

After cleaning out the dust and verifying that the power supply fan
was indeed running, I tried running FurMark again with the GTX 580 and
it ran fine! I thought I was home free until I tried running a
refresh on the Windows Experience Index and the system shut down
again. However, repeat refreshes on the Index and further running of
FurMark didn't result in any shutdowns.

Is it possible that running the computer for months (maybe over a
year?) with the power supply air intake clogged has degraded the power
supply and it is in need of replacement?

BTW, my BIOS is not set to autostart after a power failure.
Unfortunately I have not been able to make the system shutdown again
so I could check on your suggestion of observing the behavior of the
LED on the motherboard.

Jerry