I have just setup a NUC5i5RYK running Windows 8.1 x64 to use for Windows Media Centre. When it is connected to a LCD monitor, everything works fine. However when I connect it to my Samsung LCD television, if I am not in Windows Media Centre, the video is completely unusable. It flickers continuously, the open window appears and disappears, the video breaks up. As soon as I enter Windows Media Centre, it is perfect. Other full screen apps such as VLC do not work.
The unit has latest BIOS and all drives including video driver.
The initial NUC unit I had did the same thing, but it has been returned due to a detached wireless antenna cable, and the replacement unit does exactly the same thing, so I don't think it is faulty hardware.