ZX6000 hangs on boot of Debian 10.0

Hi everyone,

I'm new to this list so apologies if this is well covered ground. I
recently got a ZX6000 box (2x1.3GHz McKinley, 4GB RAM) and installed the
most recent Debian image (2020-08-19). Installation went fine, but the
system hangs at boot after GRUB loads the kernel (5.7.0-2-mckinley) at
seemingly random points. Sometimes the hang is immediate as soon as ramdisk
is loaded, sometime it makes it for ~20 seconds. Most often there is no
error message, but occasionally I see this message:
" watchdog: BUG: soft lockup - CPU#1 stuck for 22s!"

The same system runs HP-UX just fine, so I don't suspect a hardware issue.

Any hints would be much appreciated! Thanks,

John