Why does my PC keep shutting down unexpectedly?

It sounds like your power supply might not be supplying enough power. It could also be a sign of something else in your system causing the shutdowns, like overheating.

You might want to test your setup with someone else's power supply to see if that resolves the issue. It could give you a better idea if the power supply is the culprit.

I've dealt with something similar before. In my case, the power supply needed replacement after a capacitor exploded with a loud bang. I'd recommend looking into getting a new one as soon as possible, especially if your PC is shutting down that quickly during use.

Another possibility is overheating components, like your video card or CPU. Check to make sure all the fans are clean and working properly, and that there’s no dust buildup.