If it turns off by itself, most people would usually attribute it to overheating. But you can check the power button or the power button's cable and connector so see if there's anything wrong with it. If there's shorting between the metal contacts, it sends the on/off signal to your mobo. Maybe pull of the power switch cable for the moment and use a jumper switch to turn on your PC for the moment to see if it's really caused by the power button

Either that or the PSU issue if you have enabled "Boot up on power loss" settings in your BIOS. Maybe disable that setting first and see if your PC still turns on automatically