Ubuntu – Ubuntu GNOME display fails to start on boot

I have made a fresh Ubuntu GNOME 16.04.1 install through USB and there were no problems during the first boot. Booting my OS a second time throws me immediately to tty1. Here is what I have tried:

• Booting from my USB again then trying to boot from Hard drive initializes the display successfully. However, as with the original symptom, once I shutdown my computer and re-start it again throws me to tty1
• in tty1, I have tried issuing service gdm stop & service gdm start with sudo privileges. Nothing worked.
• in tty1, issuing sudo startx starts my display manager with root as the current user. Using a terminal window, restarting the gdm service as above successfully initiate the GDM with me as the current user.
• in tty1, typing startx without root privileges fails

Are there any défaux with init scripts ? I have updated my packages.

EDIT 1: Issuing sudo service gdm restart reactivates the graphical tty7 and solves the problem. However, I have to enter it every time at boot.
My guess is that there are some components related to GDM initialization that load after GDM or something similar.

EDIT 2: After being dropped to tty1, my "num-lock" flickers very rapidly for about 20 seconds. If I typed my user name and password during this period, I am logged in into the Graphical interface as normal. If not, then I have to manually restart GDM.
What is worth noting though, is that every time I change "Automatic Login" to ON, I find it OFF after the next boot.

Note: I have my / encrypted. My "/boot" resides on a separate partition