Instead of lightdm a flashing black screen with underscore

lightdm:
  Installed: 1.26.0-0ubuntu1

Description: Ubuntu 18.04.1 LTS
Release: 18.04

I expect the greeter to load every boot.

Instead lightdm only works for either one or several boots after I take corrective action, but inevitably this results in the greeter not loading - the last few times (at least) I noticed this was immediately after I did a sudo apt update. The graphics card on my Intel Core i5 system is an Nvidia GTX 970, and I manually installed nvidia-driver-415 using apt when I could not load a greeter as soon as I had installed Ubuntu.

Searching here and elsewhere, I have tried many things to resolve this, which I will outline below. If you know something else I can try - to get lightdm stable - please let me know.

I see a flashing black screen with a single white underscore top left. I can never get to a TTY using ctrl + alt + f3/f4/f5, REISUB does not work, so I hard rebooted, the greeter loads with nomodeset, and from there without logging in I switch to a TTY and I have tried what I outline below. Sometimes if I just reboot again without nomodeset the greeter will just load properly, without my changing anything.

Observing these permissions:

-rw------- 1 dusf dusf 56 Jan 7 17:38 .Xauthority
-rw------- 1 dusf dusf 15886 Jan 7 17:38 .ICEauthority

drwxrwxrwt 16 root root 4096 Jan 7 17:45 /tmp

I input the following, and could login normally next reboot:

sudo chown dusf:dusf .Xauthority
sudo chown dusf:dusf .ICEauthority
sudo chmod a+wt /tmp

The next time the issue occurred I input the following, and could login next reboot:

sudo dpkg-reconfigure lightdm

In .xsession-errors, I noticed the following line, which I thought could be relevant considering I see an underscore instead of a greeter?

_=/usr/bin/dbus-update-activation-environment

Next temporary fix:

sudo mv .Xauthority .Xauthority.bak

Again:

sudo apt-get purge lightdmsudo apt-get install --reinstall lightdm
sudo apt-get install gnome

After a sudo apt upgrade a few days ago with the same issue recurring I input all of the following and everything seemed fine for a few days:

sudo apt upgradesudo apt purge lightdm
sudo apt purge gnome
sudo apt purge nvidia*
sudo apt remove nvidia-driver-415
reboot
sudo apt purge nvidia-*
sudo apt install nvidia-415
sudo apt install nvidia-driver-415
reboot
sudo apt purge xorg-* xserver-xorg
sudo apt install xorg xserver-xorg
sudo dpkg-reconfigure xorg
sudo apt purge ubuntu-gnome-desktop
sudo apt purge gnome-desktop-environment
apt install ubuntu-gnome-desktop
sudo apt install ubuntu-gnome-desktop
sudo apt purge lightdm
sudo apt install --reinstall lightdm

The next time the issue occurred I noticed in my .bash_history the last commands I input were:

sudo apt update
sudo apt upgrade

and I uploaded the following logs:

/var/log/lightdm/lightdm.log

https://paste.ubuntu.com/p/wkfmNrrctG/

/var/log/kern.log

https://paste.ubuntu.com/p/g9bRKpMnxW/

.xsession-errors

https://paste.ubuntu.com/p/KhjrKYwmdj/

I read adding an xrandr line to some files may be the next step, but the actual lines seem to be system specific, and their are recommendations on adding them to different files.

How can I begin to resolve the issue please?