[UbuntuGnome] Dell XPS-15 9550 HDMI causes system hang

I have the 4K UHD screen version and am ruing Ubuntu Gnome 16.04 Beta1 using 4.4.0-13-generic and the Nouveau drivers.

If I plug in an exteal HDMI monitor (FHD) either into the HDMI port, or into the USB3.1-coected adaptor box that provides HDMI (and etheet etc.) then the whole system hangs, completely unresponsive and needing a hard reboot. Before hanging the 2nd display gets an image as expected, but the laptop's screen flashes randomly, sometimes you see weird artifacts. Then the hang comes.

I've also tried to get the nvidia drivers (340 and 361, 'tested' and 'updates') working, but failed. When the Nvidia drivers are installed the display manager doesn't come up and the console screen flashes randomly and won't reliably accept keyboard input. You can SSH in to cleanly shutdown if you're lucky.

I'd really like to get the HDMI working, and ideally I'd like to get the nvidia drivers working too so I can use OpenCL.

It seems like such a serious quick death.