Thanks Richard,
Doesn’t work . . .
I did it over and over again and I keep getting the same ‘error’.

ERROR: unable to load the kernel module ‘nvidia.ko’. This happens most frequently when this kernel module was built against the wrong or improperly configured kernel sources, with a version of gcc that differs from the one used to build the target kernel, or if another driver, such as nouveau, is present and prevents the NVIDIA kernel module from obtaining ownership of the NVIDIA GPU(s) , or no NVIDIA GPU installed in this system is supported by this NVIDIA Linux graphics driver release.

Please see the log entries ‘Kernel module load error’ and ‘Kernel messages’ at the end of the file ‘/var/log/nvidia-installer.log’ for more information.

You can't load any Kernel module/driver without knowing something about the sources that the module was built against. Whatever the nvidia modules are, I suspect that you're trying to load into Kernel 4.4 (the Slackware-14.2 kernel) and this is too old now. For now, install this 5.4 LTS kernel and see if nvidia.ko loads.

https://slackware.pkgs.org/current/s..._64-1.txz.html
https://slackware.pkgs.org/current/s..._64-1.txz.html
https://slackware.pkgs.org/current/s...rch-1.txz.html

Just out of curiosity: Is this a problem with the specific nVidia driver the OP mentioned? What is the latest 4.x kernel known to work?

Back on topic: I would extend the suggestion someone else made about level "3" by setting that up in the "id" record in /etc/inittab until such time as the nVidia driver is loading/working when "startx" is used to bring up X11. Once it's working, change /etc/inittab back to use level "4". If for no other reason than to avoid having to manually kill a non-functioning X11 time after time.

You can load the module at boot or do it manually. It doesn't matter either way. The point being that the module has to be compatible with the kernel.
Again, this has nothing to do with X11.

Ah. I got the impression that there was a build problem that was resulting in a bad module that wasn't loading for some reason. I've stuck with stock kernels for ages and rebuild the driver via nVidia's ".run" file when the kernel is updated. For me, working in level 3 is more convenient and then change the run level when all the sausage making is done.

Cheers...

Am I right in assuming that when you use the NVIDIA-foo.run installer from Runlevel 3 it completes without error? If that is true then it would appear the installer may be installing to the wrong kernel, not the one actually in use. It would not complete if it didn't build the nvidia.ko module and place it in "/lib/modules/<kernel version>.

So can you verify that your bootloader (or the boot item you select) points to a kernel that has the source (and headers if you don't build your own kernel) in "/usr/src/linux" and also the same kernel version in "/lib/modules"? I know it seems a bit of a pain, but I think there must be some mismatch for the module to build and install but not be found afterwards. In any case it would be worth eliminating that possibility.