Shut down problem on IMAC

Hi~ every body

I would like to ask the question for encounter problem on IMAC 5K 27-inch Late2015 SSD ver.

When shut down MAC,the general process is 2-3sec to loading then the mac will turn off;

But my problem was when i shut down my MAC, same to 1-2sec loading,

then had a lines on my screen around 2-3sec, after that the mac was shut down.

(This problem is not on every time when shut down MAC and since updated to macos sierra)

Firstly, I call APPLE hotline, they said this maybe a software problem,

so i re-install the OS X (not use time machine), but encounter same problem.

Next,on the last weekend to replace a motherboard and display card for the IMAC; but encounter the same problem too.

Anybody has this problem too? Or what can I do now?

THANKS ~!!!!!

(Photo for lines screen like with this but screen was difference line and color)