Bootloop - Wont connect to computer to restore.

Can anyone help me?

I have an issue with a 5s where it is stuck in a bootloop. It will boot to an apple logo for a couple of seconds then turn off and repeat. The odd time when its turning off i get a red screen but not everytime.

When i attempt to connect it to itunes/computer it wont connect. Its like its being turned off completely even when i try to put in recovery mode or dfu mode.

I have tried 3 different screens, a good few charging ports and home buttons. I have changed the board into a couple of different housings that i have spare.

I have also replaced the battery with a couple of them that i had brand new.

I have also had the board under a microscope and done a full inspection but found no damage to any components. I know the history of the phone as i used to use it so i know the board was kept well.

I have a feeling it may be to do with the battery but the battery has been tested with 4 other 5s and it works perfectly.

Can anyone help me on this? I cant find any similar problem on the internet and ive never came across a bootloop where i cant connect to my computer.

Thanks in advance :)