Why is my iPhone 5S rebooting every few minutes?

Hello there!

I have bought a cheap used iPhone 5S, with a damaged screen and a supposed "dead battery".

I had a spare screen, so I have already replaced that part, but I suspect the dead battery issue is not what it seemed to be. The phone had almost no charge, and rebooted every 4-6 minutes, even when plugged in to a charger. After replacing the battery it continues to restart. Also, the device charges both the old and new battery fine. So I think the device has board level damage, but I don't know what can be the suspect.

Summarized:

phone reboots every 4-6 minutes even plugged in, but charges fine

after reboot the phone shows 1% for a minute or two before figuring out the actual percentage

restores with iTunes no issues whatsoever, can activate fine

problem remains with new dock connector, new battery

no bluescreens, I dont think its long screw damage

connected to a pc, the device does not give any battery cycle or battery life info with both batteries (3utools)

Can this issue be a dead tristar ic problem?

Thank you,

Joseph