Caporegime
Not entirely sure as i used to get artifacts at that core speed. If i could keep the gpu cooler at those clocks it might be possible. So far it seems before the drivers i need 1.194v for 1125/1500. That's now come down to 1.181v.
I used to get artefacts in Heaven 4 at 1150, just in the chimney smoke. not in any game, just Heaven 4, now it fine at 1150 but i get the same artefacts at 1200, like you it seems to be something to do with temperatures, when it gets to about 74 / 75 is when it starts.