Galaxy S4: Shutting down

Hello guys, first forum post from me.

I've experienced in the past week that my Samsung Galaxy S4 shuts down all the time. When I try to start the phone again, it just shuts down again. If it's in the charger, it's working fine all the time. And yes, I have tried factory resetting my phone and it is charged to 100%.

I have been searching for an answer, and seen other people experience the same, but nothing official from Samsung yet (AFAIK):

Any of you experiencing this happen to your S4? It's freaking annoying, and it's a great phone - but this just annoys me. Not sure if it's a battery/overheating problem or if it would work to install a custom ROM?