Hi there, first test an external monitor and see if the flickering follows (if you don't have a way to do this, that's okay, but it makes it easier to rule out gpu/board issues). If it follows to the external monitor it is most likely the system board, however if it does not, you can then try reseating the display cable. Here is a link to the Service Manual that will detail the steps (at this link you will need to click on Service Manual, which will open a PDF and have step by step instructions on the display cable removal etc). I would recommend reseating the cable attached to the system board after following the steps listed and see if it corrects the issue.

If this does resolve the issue, you could also try diagnostics to rule out a screen setting, or see if the flickering happens outside of Windows, but if the reseat of the cable does not resolve it, then this would sound like a faulty display to me (possibly a cable, but most likely the display). The Service Manual will also detail how to replace this and even the system board. 

Best regards,

Brad