I have 2 4K monitors (each running 2560x1440 res). Main (samsung U28E590D) is connected with DP, second (LG 24UD58-B) with HDMI. When I play a game (XCOM-2) on main the second monitor with randomly goes blank for a few seconds. This did not happen when I had a 1920x1200 second monitor connected with DVI. Running driver 18.6.1. I have not noticed this behavior when not playing the game, but I've only had the new monitor for a few hours. Going back to driver version 18.2.1 caused the problem to occur less frequently, but it didn't go away.
