I own an ASUS KJ550X laptop, the GPU is an NVIDIA GEFORCE GTX 950M (together with the Intel integrated one) the processor is an Intel i7-4720HQ. I have installed Ubuntu 14.04 in dual boot with Windows 10.
In order to optimize the usage of the GPU, i switched to the Nvidia proprietary drivers (version 346.96).
Now, when I plug an external monitor through the HDMI into the laptop, both screens (external monitor and laptop's one) go black and start blinking really fast. This issue was not there when I used the X.org drivers, but I need to use the proprietary ones (the external monitor used to cut the edges of the screen with X.org).
Can someone help me? I'm sorry for my bad grammar, english is not my first language, I hope everything is understandable
EDIT: I tried logging in as a Guest user and evrything worked, the screen didn't blink anymore, I don't know why