Why when I switch it on it goes off again

Morning from Australia!

PS4's are very well known to fail, and as such there's plenty of video footage documenting what happens & how people fix it.

Just checking, when you say it goes on & off, what specifically does it do? Simply because on & off can be multiple different things on PS4's.

Also, have you tried taking it apart? I know repair shops tell you not to disassemble, however as long as you follow the guides on here you shouldn't break anything & you can try find the root cause of what's going on, if a component has failed on the board.

Let me know when you can :)