On 2/19/2017 at 9:14:04 PM Paul wrote:

tb wrote:
I have a desktop PC with Linux MX16 (64-bit) installed.

I have noticed in the last few months that occasionally Firefox
freezes. In a couple of instances the PC just shut down on its own.

This evening, all of a sudden it started shutting down and
rebooting in rapid succession.

I pressed and held the power button to shut ther PC down completely.
After a few minutes I rebooted and the PC has been working ok so
far.

Time to replace the power supply, maybe?

What do the experts think? Hopefully it is not something that has
to do with the motherboard. I can replace a power supply but cannot
repair a motherboard...

I would want to understand first, exactly what
kind of shutdown it is.

My PCs are set here, to go OFF if the power fails.
So they won't start booting again. If the main
rails +3.3V,+5V,+12V were to go off, then the PC
would not restart immediately.

The CPU has THERMTRIP. That gates PS_ON# and
turns off the request for power to the PSU. That
shouldn't restart on its own either.

At least some of the monitoring methods, could be
using +5VSB to "latch" or "remember" the failure. Say
for example, VCore had an overcurrent detection feature.
Maybe +5VSB would power logic remembering the failure.
Only fully turning off the PC using the switch on
the back, waiting 30 seconds, turning it on again,
would make it "ready for another boot attempt".

AC ---- rectifier --- cap ---+-- Main_PSU_section == 3,3V, 5V,
12V etc |
+-- Separate +5VSB for sleep, soft
power button, +5VSB lights the
mobo green LED.

The PC does have a shutdown and restart sequence,
but normally some code in the operating system requests
that.

Maybe an indication from ACPI Power Button,
might trigger an OS shutdown (but not a restart).

I know you're in a rush to "fix something", but
I'm trying to save you some money. For example, if
you change the PSU and it isn't fixed, you're not
going to be very happy. So first, try to describe
what the shutdown appears like - is the timing the
same as if you selected restart from a menu ?

Have you tested with the LiveCD of another distro,
to see if it behaves the same ?

A bad PSU can make noises (or a puff of smoke) at
startup. The CPU can crash, there could be a Linux
kernel panic, and so on, if the +12V isn't stable.
To shutdown and restart, seems a "bit too orderly"
for me. Like this was coming from software somehow.

Paul

I'll try to better describe what happened. Please keep in mind that I
am not a techie.

The shutting down and restarting was not the normal shutdown and reboot
that one might do using the shutdown/restart software. It was as if I
was playing with the power button of the PC: turn it on and off in
rapid sequence. The green LED light for power on the PC chassis kept
turning on and off. It did that several times. I know that there is
nothing wrong with the power strip (or the power supplied to the strip
from the wall plug) because I have a couple of other things connected
to it which were not affected by this issue.

This is the first time that the PC does this on/off behavior. A few
times in the past, the PC completely went dead -- all of a sudden!

In all these instances, letting the PC rest in the off state, waiting a
few minutes and then turning it on solved the issue.

I used to have Windows 10 installed on the PC. And a few times the PC
completely went dead without any warnings, too.

In my limited tech experience, I suspect some sort of hardware problem.
I'm hoping it is the power supply and not the motherboard itself.

--
tb