One of the main difference between the official Ubuntu 16.04 and 18.04 release, in term of "login screen / shutdown / restart" area known by my "developer background not that much interested by the linux world" is the change of the display manager.
Ubuntu 16.04 use lightdm.
Ubuntu 18.04 use gdm. G for gnome, as the official Ubuntu 18.04 now use gnome by default. Also some flavior/variant like Xubuntu 18.04 still use lightdm.
I give a try to returning to what worked for me before using Ubuntu 18.04 release, so using lightdm like Ubuntu 16.04 over gdm and it solve the problem. When installing lightdm I get a window/prompt in the terminal asking in a "ncurse graphical interface" if I want to use gdm or lightdm, with the down-arrow I choose lightdm. I reboot the computer and I can now bootup and shutdown my computer. This is wonderful.
sudo apt-get install lightdm
As it freeze on the login screen randomly, I install lightdm in the recovery mode. Holding the shift key right after starting the machine should display the grub menu. If not press the escape key once.
Then you should be able to choose in "advanced options" to boot in "recovery mode". If you press it more that once it will go to the grub console and that's not what we want.
Don't forget to choose lightdm over gdm in the graphical prompt while installing lightdm.
I also find this topic but it seems longer to apply a fix and not sure it will solve the problem for me as my CPU is a Intel Skylake family member and not Intel Bay Trail:
I was personnaly thinking that it was all a typical NVIDIA Driver problem but it appears that it can be anything. Computers...