For your information.
Performed a fresh/clean install from the live CD of 20.04 MATE and chose to install third party drivers.
The install went smoothly and I can login. I can confirm with nvidia-settings & nvidia-smi that the nvidia 390.132 driver is correctly installed. System has nvidia GTX675m (Fermi)
However!!!
Opening just about any window causes instant black screen and the Xorg xserver goes 100% CPU.
Even trying to use the menus on the terminal app causes the black screen.
The only access after this point is via ssh. The Xorg process cannot be killed (-9) and the system indefinitely hangs if a reboot/shutdown is attempted.
The message in the log is the : GPU ... has fallen off the bus
I have submitted nvidia-bug-reports to nvidia dev forum but I see this as a Linux kernel issue as the driver hasn't changed in years. I also don't really see this as a MATE issue.
However thought others may need to be aware if they are having similar issues.
The same system was working fine under 18.04 MATE with driver 390.132 and still works under Windows 10 with same driver (dual boot, separate drive).
Hi Walter,
try removing the 390 driver, see the following guide and the section about removing Nvidia drivers. (Read from: Okay, I installed the wrong driver and now I can't boot/my graphics are shot!:) 
Thanks for the reply.
I have already tried running on the nouveau driver and that works fine. However the nouveau driver has no support for Fermi cards (support ends at GTX 400 series) so the driver is completely hardware un-accelerated for GTX 600.
The performance impact is quite substantial and I don't find that acceptable.
To make the point clear I can game on 18.04 but now on 20.04 that is no longer an option! There is no point keeping Steam installed.
At least the system (nvidia driver and Steam) still works on Windows 10. I really can't expect the hardware to be supported for ever. Those new ASUS G14s look really nice. I'll just have to spoil myself!
Anyway, repeating myself, I don't see this as an nvidia or MATE issue. I'm not really really expecting a solution from this forum.
This is just a For Your Information post.
Walter's post is an interesting one in that it may explain my problem with 20.04.1. (The problem is that it hangs on shut down.)
The installer would not finish if I chose to install third-party drivers. It would only finish if I didn't do that (and this turned out to be a known bug with the 20.04.01 installer).
Basically everything is fine with my 20.04.1 install (it's on an external drive) except that it won't shut down.
(The error message I get is: DRM: GPU lockup - switching to software fbcon.)
I am using a somewhat older graphics card: the NVIDIA GF106 GeForce GTS 450, with the 390.138 driver, but it's worked fine so far.
The driver is the correct one for the card, and the card, I believe, is the newest one (or among the newest) that my machine will accommodate.
If I could get 20.04 to shut down properly, I'd be fine. (It does reboot with no problem.) My card and driver work fine on Ubuntu-MATE 18.04.
But from reading Walter's post above, I'm starting to think that 20.04 has left the 390 driver behind. Or the latest kernels, or whatever, have left it behind.
Does anyone know anything more about this? I explain the situation further in a recent post here.
Hi WP,
I would just let the OS select the best driver rather than pointing your system to a particular driver set, the onboard drivers should be more than enough for such an old GPU?. 
What do you do to "just let the OS select the best driver"?
I've used both nouveau and nvidia, neither allows my 20.04 .01 installation to turn off the computer upon shutdown. I have to get up & lean down and hold the power button in for several seconds to turn it off. That's with trying to shut down with either of those drivers. I have 20.04.01 on two separate external drives, same thing with both. I probably need a new nvidia driver, which means I need a new GPU, which means I need a new computer - I got it in 2009, after all.
Hi watchpocket,
you don't need to do anything as during the installation, the OS will install (should) the best drivers for your GPU!. If as you say, you have a PC from 2009, you might consider an upgrade if you can afford it!. 
I don't use hibernate of power saving as I simply switch my computer off when not using it as I have often had problem with sleep etc, even on Windblows 10!. 
Thanks for responding, but the installer wouldn't finish unless I specifically UN-checked "third-party graphics" before running the install. My card and driver work fine with 18.04, but with 20.04, as mentioned above, everything is fine except for not being able to shut down. (I never hibernate, either, btw).
Anyway, for a whole host of reasons I've decided to get a new computer (or rather I'm building my own).
Keep us posted in case you need more help please!. 
