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