Can boot into 32-bit but not 64-bit live session

[Agapanthus]

I want to try out Lubuntu on a Samsung laptop with Nvidia graphics. The 20.04 image boots to the selection menu, but when I try to launch the live system, the computer eventually reboots and starts over. The 32-bit version of 18.04 is able to launch the live system, the 64-bit version of 18.04 keeps rebooting, like 20.04. However the laptop should be capable of running a 64-bit instruction set.

Specs:
Samsung Aura P8400 Madril (R560 Series)
CPU: Intel Core 2 Duo P8400
Graphics: NVIDIA GeForce 9600M GT - 512 MB
RAM: 4096 MB, PC3-8500F

1. I have checked and rechecked the ISOs and DVD/USB media
2. I have tried “safe graphics” and “nomodeset” as well as other switches (via f6)
3. The blue Lubuntu screen only displays the moving dots animation and in certain cases does a system test, which it passes.
4. Have glimpsed a very brief message (on black screen) to the effect that “no caching mode page found” – which I understand is normal and no cause for concern.
5. In one case brief glimpse of “Starting hostname service” and something about “terminate Plymouth bootscreen” before system reboots. (i.e. not freezing)
6. Have not found a way of pausing the computer to peruse the screen output, and since the live system is presumably installed in RAM, there does not appear to be a log file.

Being new to Linux, I am not sure where to go from here. Any pointers would be appreciated.
Thanks

[Agapanthus]

Update:
Booting the 20.04 live system, I have now activated all but the first and last options under f6, added vga=771 and deleted “quiet splash”. On booting into safe graphics I (inelegantly) used a camera phone to capture the screen output.

The system checks are all okay and the boot proceeds with green [OK] messages until it reaches this line:

[ 177.701323] iwlwifi 0000:02:00:0: pci_enable_msi failed - -38

Not sure whether this is causing the “crash” or whether the next command brings the system to its knees. My hunch so far is that the difference in behaviour between the 32-bit and 64-bit system is driver-related, but I would be very grateful for the advice of those who are more knowledgeable.
Thank you

[Agapanthus]

From watching the screen output displayed before the boot process is interrupted, it seems that systemd is mounting root partition, checking file system, initialising network services etc. There is also a message indicating that the display manager is being launched and this is when the screen blanks.

So what is the difference between launching the display manager in a 32-bit system and a 64-bit system? Why would this be successful in 32-bit lubuntu 18.04 but fail in 64-bit lubuntu 18.04 (and 20.04)?