iPhone 6 Apple logo reboot, when screen is put down (original)

Hey, I did get a free iPhone 6 that randomly started do the Apple logo reboot.

I found out that if I connect it to a computer it starts fine and works well and if connected it never reboots.

But if I unplug it, it starts rebooting after 5min-3hours.

Can this be a U2 Tristar problem?

I have opened it up and it's no sign of water damage, and tested some parts in diod mode but haven't found anything wrong.

I have also reset it and updated it.

And replaced the battery

I replaced the charging contact flex last night and it's been alive for about 8hours now.

Can the charging flex cause that kind of problem? O.o

When I did have the charging flex connected but the screen "open" it never died. After replacing the charging flex and putting down the screen it started rebooting again..

When I get home I will test if it will be on after opening the screen again.

Can it be some sort of ground shorting somewhere?

I talked to the last owner, a month before this problem came she had got the volume button flex replaced by an repair shop, and the problem just came. Diden't get dropped or anything just before and no sign of water damage.