LG G2 D802 Phone shut down/Bootloop

[xrionitx]

Hello, i had been using Lg g2 d802 smartphone since a year, recently while using it, it suddenly switched off. Then after i tried to restart it, it took many tries to switch on. Then after 10 mins it switched off again.
I showed it to a mobile repair guy, he told me that hes not sure enough, but maybe the Charge flex cable have stopped working, or the motherboard.
So i'm sure what the issue in these two is true..

 

[rgd1101]

MERGED QUESTION
Question from xrionitx : "LG G2 Phone Rebooting"

Hello, i have been using LG G2 D802 phone since a year now, but a week ago the phone suddenly switched off while using. Then i tried to restart it, it did get started for around 10 mins, i did a factory reset, then afterwards it switched off again, then after trying many times it gets started for just a min, and then turns off, i asked a phone repair guy, hes not sure if the Flex cable is not working or the motherboard.. Can anyone please help me with it, whether i should change the Flex cable of the phone or the motherboard. or what the issue is...