iPhone 5s restoration - why does it keep rebooting?

Hello

I’ve recently purchased an iPhone 5s with a bloated battery in order to restore it. Long story short, a friend of mine really wanted to change the battery himself ( i bought the battery from him ) and didn’t let me do it.. he broke the already battered screen completely.. resulting him in giving me an iCloud blocked 5s as a spare.

Transfered the screen from the iCloud locked phone as well as the battery to the one with the good motherboard. It all seems to work perfectly on the phone, even the battery can hold a charge by what i see.

However, the phone keeps rebooting itself every 2-3 minutes of usage, even on idle sometimes ( screen off, not using it). When it does turn off it starts by showing 1% percentage battery but turns back to normal after 1 minute. I tried tracking down the problem, but i dont have any spare display or battery.. I tried disconnecting the camera and earpiece module, i’ve tried disconnecting the touch id which also doesn’t work.. I’ve tried restoring it.. Also i’ve tried fully draining the battery and recharging it. It stayed at 8% for hours but after a restart it turned back to normal.

It runs the last iOS, it’s a 16 Gb one, everything seems fine except the reboot.

Any ideas ?

Thanks in advance