The phone keeps restarting in 3min intervals and heating

Hello,

i got a dysfunctional Iphone 5s from my friend, who said he couldn't use it anymore because it kept restarting. Formerly he thought it was a battery, which we changed. But then it was getting worse, it was uncontrollable, it was just possible to extract the data via cable.

Now what it does, restarts in approximately constant 03:30 sec intervals, while the bottom right quadrant(viewed from the side of the display) is getting considerably hot over time. I can still operate it within the boundaries of locked Apple ID and factory reset(set up a wifi connection etc)

I can imagine there is a hardware issue, but does anybody has a clue which part is to change ?

any advice will be appreciated.

thanks.