Computer doing sporadic hard-shutdown?

It does sound like a power source issue, but before I would do that, I would power it off completely. Than I would disconnect every single device connected to the computer. Than I would wait for a few minutes, at least 2-5. Than I would reset the PMU or SMC if your computer has one. Some computers PMU/SMC is reset just by doing what I just suggested, but others have a button on the logic board it may be on the logic board as well, you'd wanna check based on the model.

Once you reconnect the keyboard, mouse and power, and restart, if you are still shutting down with nothing connected you have narrowed to definitely the computer. Also it may seem stupid but check connections on power connections. (i know...) But if it doesn't shut down anymore than it is a peripheral possibly. Have you changed any hardware of software recently, as in right when this started happening? Could add to it. You are probably right but it does not hurt to check and only takes a few moments.

It could be the power supply, but first you should rule out an overheating issue. If your computer is getting too hot, it could be shutting itself down to protect its components.

Make sure your computer is in a well ventilated area, the fans are working properly & heatsink are clean of dust, and air inflow openings aren't blocked.

It's either the power supply unit, or the processor is over heating. You may have to pop open the case, turn on the PC and do a visual inspection to verify that the CPU fan is spinning. If it is spinning it may not be spinning fast enough to keep the processor cool.

I don't know what you mean by a "power source" issue, but it sounds to me like it's a power supply issue inside the machine. You may need to purchase a new power supply and replace it in your machine (or get it serviced).

The first thing to try is to plug it in somewhere else to see if the same issue occurs.

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