I finally fixed this. The problem was on the Freesync and 75HZ refresh rate monitor which are probably not compatible together in Mac. I thought about it because the guy which has uploaded that video on youtube has the same monitor as me.
So if you are having this problem, turn off...
I have a problem with my new Sierra Hackintosh that I'm building.
First of all I have to say that my machine is powered with the new Intel i7 8700 with the Gigabyte z370 HD3P (but actually I think that the problem that I'm having is not because of the incompatible CPU.)