IMac screen problem

I have a 2009 iMac which I have installed Manjaro 20.0.3 XFCE to as a dual boot. (Specifically Apple iMac 21.5-Inch “Core 2 Duo” 3.06 (Late 2009))

It does not handle the screen correctly. I think it is a PCI problem.
Right at the beginning it gets an error PCI -61 (it’s not there for long). It continues on booting and then the screen goes ‘funny’ - it is obviously addressing the screen incorrectly - it shows the screen divided into 4 squares all identical very whitish and hard to read.

The live disk works fine, although mhwd seems to take about 5 minutes. Manjaro installed on an external hard drive (on another computer some time ago) works fine. My copy of Porteus works fine. It’s just this hard disk install that doesn’t.

It’s a fresh install, so I can’t ssh to it. The screen problem is prior to the graphical login so Ctrl-Alt-F? does not help.

I can boot with Porteus or the live disk, but where do I look for clues or make changes?

PS. It’s after 9pm here and I have an early start tomorrow so probably won’t see any replies for a while.

see on Grub screen
type videoinfo to see results that can be used

I don’t get grub. Using reFind.

I think reFind is the problem.

I installed grub but it does not appear. Meanwhile reFind is also a boot loader.

I believe it has a configuration file. I probably need to find that and give it the right switches for starting the kernel. I probably also need to uninstall grub.

Do command+R option+P at boot up. Login to OSX and reinstall reFind. That gets mine back to working. If you mess up your grub partition, or reinstall another unix base. If any thing bugs out that is first to crash. Most time you just reinstall reFind and it should find your partitions back.

I can boot the partition with Manjaro in it but if I do it through refind it gets a PCI error -61 and then can’t do graphics. It works fine using a BIOS type of boot.
I tried creating a second instance of Manjaro. When I selected the ‘efi’ version of the installer the live iso exhibited the same problem.
When I boot the original install via refind and specify nomodeset it gets further but can’t run the display manager.
I can use it but it’s a pain to boot! The machine won’t boot from a USB either!
I boot from a Porteus CD, select the plop boot loader, create a profile for the partition because plop does not recognise it, go back and select it then plop complains about MBR but let’s me boot anyhow - after that it all works beautifully!!