Ubuntu 16.04 VM Guest - Boot on TTY

On my VM guest Ubuntu 16.04 (host Win10), X server seems to fail start. The tricky thing is that installation + 1st reboot are OK, it's only on the next ones that I always fallback on TTY and console prompt. Furthermore, while arriving to login prompt, screen seems to flicker.

startx command fails with "coection refused". I saw several posts with similar problems, especially with nvdia cards (mine GTX 960M), but so far, I troed to reinstall both "Nouveau" and nvidia-current for proprietary drivers, without success.

Anyone got a lead to fix this ?

Many thanks,

Nicolas