Why is my Galaxy S5 is inconstant reboot mode?

[AC Question]

I turned my phone on, and it booted up, yet as soon as small screen for checking I have closed my case appears, my phone reboots. Can this be a battery issue? I tried a Safe Mode reboot, yet it still shut down. It doesn't stay on long enough for me to get into settings. Help me please.

 

[anon(632115)]

No, it's a sticking power button