I had similar things happen when I was using Wayland, but it wasn't enough for me to just change the gear icon to X11/Xorg (I think the login screen still used Wayland, which still caused flickering -- sometimes until I rebooted!)
So, edit a config file like so:
sudo nano /etc/gdm3/custom.conf
Then, uncomment [remove the # from] the line containing:
WaylandEnable=false
Save it and reboot.
I hope it helps!