iPhone 5S - Random / sudden reboot

[Thoma5]

my iphone 6 aso start to kana but then once in a while nia...zzzz

 

[thenakedshades]

I dun have many apps running in the background. I usually kill task when i have the free time.

i always kill apps before i press my power button. A habit liao

 

[Okane]

I encountered this once yesterday and once today
Not sure is it hardware or software