Posted July 06, 2016
What's I've tried:
CPU temps are very good when it shuts off (under 40 C). So, it's not overheating. Heat sink and PC are clean and dust-free.
The power supply is good (still happens with a new power supply)
All cable connections are secure.
Memory is good (I ran Memtest86 for half a day and got zero errors)
Memory voltage is good (1.536 v)
I updated the bios - still happens
I downgraded the bios - still happens
It happens in Linux, Windows XP, and Windows 7 (so, it's not an OS or software issue). Even still, all power management settings are disabled in all operating systems.
It also happens regardless of which hard drive I use.
House's power is good (everything else stays on).
The outlet is good (other pcs work fine on the same outlet). Other items also stay on when the pc shuts off (like the monitor).
Motherboard is not bowed. The motherboard and case were designed so the case supports the weight of the heat sink.
So far it hasn't shut off if I stay in a pre-boot environment like the bios or when running memtest86 (at least, it hasn't yet).
So, what now?
Let's assume for the moment that the motherboard and cpu aren't failing. What else can cause the computer to randomly shut off?
edit:
One thing I've noticed is that if I leave the computer running with an OS booted, but not doing anything, it can go several hours and doesn't shut off (yet). It only seems to happen while I'm doing something. However, I can often work actively or play a game for several hours and it won't shut off. It seems completely random.
edit 2:
Also, it's not malware. I use linux to avoid that and I keep windows offline always. I only install programs in windows that's I've downloaded in linux that also pass a complete scan from the virustotal website.
First, check the connection from your power supply to your motherboard. Unplug that connector and check both the male and female connectors for any cracks or discoloration.
Also, check the connections for your power switch and reset button, perhaps there is a circuit in either of those that is getting opened (turned off) due to wear since the power button is one of the most exploited switches on our PCs. Same goes for the reset button as a just-in-case thing.
You can try locating the cables going from your PC's power switch (closer to the power switch) and wiggling them gently to see if your machine turns off. If you have easy and safe access to the cables going out of your power supply and to the main motherboard connector you can try wiggling those gently while your PC is running but this is kind of risky since most PC cases these days tend to be cramped and have all the important bits deep inside the case.
I would also do the smell test. Open up your case and see if you can smell anything burnt. Granted, this is kind of hit and miss but it helps to have every angle covered :)