Problem booting Mac Pro 5,1 with linux kernel 6.8.0 – no problems with linux kernel 6.5.0

cross-posted on Macrumors forum and Ask Different stack exchange - no responses.

I am running a MacPro 5,1 which can triple boot MacOS Sonoma, Ventura and Ubuntu/Linux from separate SSDs. In the past two days I could not boot Linux 6.8.0. MacOS and Linux 6.50 boot fine.

I use OpenCore Legacy patcher (OCLP) on the MP 5,1 to select the required boot drive (each OS on separate SSD).

I do the following to attempt to boot Linux:

• Select the Linux boot drive from OCLP menu.
• When screen clears press ESC once.
• The Grub 2.06 menu appears
• Select Advanced options

Two Linux kernels, and recover mode, appear for selection on GRUB2 menu

1) If I select linux 6.8.0-40-generic and press ESC to display boot message the boot gets to various points then hangs. A regular sequence of messages includes: Mounting bootefi, … Finished load kernel module efi-pstore. At this point the boot does not proceed, the machine hangs for about 30 seconds then the screen goes blank.

The fans ramp up speed for 2 seconds then stop and the machine just displays a blank screen. I have to press the power button for about 5 seconds to switch off the machine.

2) If I select linux 6.5.0-45-generic the machine boots fully, I can sign on and all attached screens display.

3) If I select linux 6.8.0-40-generic (recovery mode) the machine boots fully, I can sign on and one attached screen displays – the other screens are blank.

Any suggestions on how I diagnose and fix the 6.8.0-40 boot problem? I can work perfectly fine with 6.5.0 but the boot process is inconvenient.

Thanks