NetBSD 8.1 / Sun Ultra5 install crash during initial kernel device autosizing

Hi folks!
  I have a 360MHz CPU, 512MB RAM Sun Ultra5 that, when booting from
the NetBSD 8.1 install CDROM and working through the kernel's initial
autosizing of devices, gets to mentioning the discovery of its m64
graphics card, very briefly distorts the screen as if changing into
some other frame buffer mode and abruptly goes black right then and
there.  Even the monitor (Sun GDM-21) switches off and there's no more
activity from the cdrom (which leads me to believe it's crashed,
though I haven't tried a serial console yet.

Is that a known issue?  Any guesses known workarounds?

thx
jake

P.S. It does the exact same thing on my other Ultra5 with 256MB RAM,
so unlikely just a broken hardware.

P.P.S. My Ultra1 and SPARCStation 10 work very well under this same OS
release (and with this same monitor) and don't crash when autosizing
their graphics cards, though I do notice a font change happening
around that time in the boot process.  Maybe related?