I am having trouble getting my external monitors working through the surface dock. They flash on and off. Sometimes they work for a minute, then they start flashing off and on again. Either one connected directly to the surface works fine. I'm trying to set them as extended, with the Surface Book screen set to off. I'm using MS recommended cable. One monitor is LG ultra wide 2560 x 1080 connected via HDMI. The other is QNIX 2560 x 1440 concocted via DisplayPort. I've tried three different Surface Docks. Any advice?