I got the same problem on my Gigabyte Notebook. It has a GTX 860M Maxwell and a 4710HQ. When connecting an external monitor via HDMI DX11 games are switched to 1080i. The horizontal refresh rate of the monitor is then reported with 34kHz instead of 78. It flickers and fps are bad. Didn't find a workaround yet.
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Any other resolution but 1920 x 1080 is diplayed correctly.