Your symptom sounds like;
	* hardware fault
	* broken I/O device driver
	* (weird particles shot from outer space... :-> )

Just to report my case;

Although I don't have any PMAX other than several MAXINE (DS5000/25),
I have never experienced kernel troubles with stock NetBSD/pmax
(formal releases and snapshots) since NetBSD/1.1.

> Now I'm having another problem: Though it works generally, for some
> reason or another, my DECstation 3100 (KN01, Version 7.0, with two RZ55
> disks, CFB, 24M), crashes when it's doing something - anything,
> randomly.  It just sits there, frozen.  No panic message, no savecore
> dump.  I haven't been able to compile a kernel since it would crash in
> the middle somewhere and I'd have to restart compiling from the
> beginning (is this normal???  Do I have hardware trouble?)
>
> It will always take a while before it crashes (around 30 minutes to few
> hours), and never when it's idle.

Tohru Nishimura
Nara Institute of Science and Technology