[a6491]

Hey guys, having some issues with this S4. The phone was happily charging. Next thing you know, it turned off. Upon turning it back on, it just wouldn't. The phone was perfectly fine prior to this issue, hadn't been dropped, no water damage.
What I'e done to fix the issue:
Replaced charge charger port
Replaced battery
cleaned connectors thinking it may be connectivity issue.

What I've achieved so far:
So far I can only get the phone to turn on for a few seconds ONLY if I take the battery out and put it back in. Without pushing any buttons, it automatically turns on. I can get into recovery mode, for a few seconds before it turns itself off. The following is a video illustrating what I mean.
/watch?v=ksVV7e4JgpU&feature=youtu.be
It's a youtube video, append the link at the end of the URL

Excuse the audio.
any suggestions as to why this is happening would be greatly appreciated before I waste money and replace the motherboard itself hoping that would fix the issue