Help Galaxy S5 (SM-G900I) randomly turning off

[Kalyori]

I am having an issue with my Galaxy S5 (4 years old) where it randomly turns off. I can't turn it back on, either... I have to plug it back into power and then once the battery capacity symbol shows up then I can turn it back on.

I can be using it one minute, put it down and come back half an hour later and it's off.

I asked about this elsewhere and it was suggested that it could be (or was highly likely) an issue with the battery. Consequently I bought a new one ($50 AUD - from a battery store - not eBay) however the issue persists.

I can't remember the issue happening prior to updating to 6.x, so perhaps it's worthwhile rolling back to 5.x?

Does anyone have any ideas what this could be? Any suggestions as to what I can do to try and diagnose it?

 

[PitCarver]

Is it actually shutting down, or is the screen simply going black?

My old S5 would black out for no reason.

 

[Kalyori]

Is it actually shutting down, or is the screen simply going black?

My old S5 would black out for no reason.

Well, I think so. No buttons work whatsoever, and then when I connect it to the power it shows up with the battery capacity symbol (like when it's been turned off, not at the lock screen). And then I press and hold power on and it turns back on.