I have a gtx 1080 running with an i7 6700k on an Asus Z170-A board and it only posts with onboard graphics. Every time I use my 1080 it will make a signal to my monitor but only to a black screen. I've already tried in one of my friend's computers and it worked fine. I've enabled iGFX and then installed my GPU and I got a Code 43 which mean Windows has stopped the device because it has reported problems.