This is becoming a common problem now. It is most likely the GPU Buck converter U8900 on the motherboard and is an issues with the tin solder used by the manufacturer separating from the chip due to heat stress over time.
This is most likely a logic board issues and can be resolved if you have a very nice soldering iron and microscope. Take a look at this video for how to solve it.
https://www.youtube.com/watch?v=BYLuJBNLHSU
Hope this helps! Best of luck