I have been trying to run Ubuntu 20.04 on an old iMac (late 2009). With a fresh install, it randomly freezes once resizing windows, changing the background wallpaper, or while opening a browser.
I have read a few posts describing the same problem, pointing to changing the graphics driver to the legacy-nvidia driver:
Help needed with Ubuntu 20.04 on iMac late 2009
Following the first part of the post:
sudo add-apt-repository ppa:kelebek333/nvidia-legacy
sudo apt update
sudo apt install nvidia-340
sudo reboot
It seems to work fine at first, however, I can't add the brightness control described in the linked topic. The monitor will stay black on the next reboot.
I have the same issue the monitor stays black after a few restarts.
Could you give a Linux beginner some pointers?