This has been going on for about a year now but I am now attempting to do something about it.
On two separate systems, one Intel I7 and one AMD Ryzen 5, with plenty of RAM, I have the same thing happen. After a few times booting, the Nvidia Driver seems to stop working. I have tried a number of Nvidia Drivers, like the x86 64-470 or other versions like the 45* or 43* series, but they all do this. In addition, the two systems have different graphics cards, one is a GTX1060 and the other is an RTX2060.
It seems that the only point of similarity is the Ubuntu 18.04 LTS. I have never done an "upgrade" but have worked with the same software for over a year.
When the driver stops working, I check to see if I have already blacklisted Nouveau, which I have. I change to "runlevel 3" which is "sudo systemctl set-default multi-user.target" and then I run the driver (it complains otherwise). After that, and I get no really bad messages from the driver install, I go back to runlevel 5 "sudo systemctl set-default graphical.target".
That seems to work. Until it does not. I think that I never last more than 3 or 4 reboots before I have to redo the process.
Does anyone have any idea of how to fix this?