Question My Brothers PC keeps shutting down on its own.

[Silas03]

So my brothers PC keeps shutting down on its own. When it does this it displays the normal shutting down message. He says it also turns on by itself. It shuts down whether its idle or in game.
The specs are very minimal;
i5 2500
gtx 1050 2gb
16gb ddr3 ram
265w psu that came the the dell mini tower (tested with a different psu, faced the same problem but not sure of the quality of the psu its a random one)
dell optiplex 990 mini tower case.
1tb 7200rpm hdd
Ive tried updating the bios to dells A24 and turning off fast start and things like that.

 

[ohio_buckeye]

Almost sounds like the power button might be sticking somehow. If you would press the power button on most systems, it will give it a command to shut down.