I have just upgraded Ubuntu from 16.04 to 18.04.2 LTS on my PC (Intel i7, NVD9 GPU, Gnome). The problem was not present on 16.04. Now the screens flicker, especially when there are large blocks of black or blue. It does not render them useless, but is annoying and makes it harder to stare at the screen for hours every day.
It is much more intense on one of the screens (I have two, both connected with HDMI to the GPU).
I can't use Nvidia drivers as they just don't work with my NVD9 GPU, so I am sticking with Nouveau X.org.
I have tried changing refresh rates in /home/USER/.config/monitors.xml (they were at 59.XXXXX, I changed it to 60), but it does not seem to work.
I have made some changes to the settings of the monitor itself, reducing the ammount of blue light helped a bit, but the problem is still here.
Any ideas about how to fix it? I am pretty sure that it's a matter of the OS/drivers than the hardware itself, as all worked just fine on 16.04.