@privateproperty all of this sounds like a battery issue. If nothing else, I would replace it so that you know you have a know good place for further troubleshooting. Remember that the battery may charge but may not hold a charge for long and fail under power consumption. Use something like [https://youtu.be/ydSYxG30IZw|this video] to work on your robot.
While you work on your robot, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy!