Under a load (compiling mh, downloading tk today), my sun3 seems to
die with an MNI interrupt received:

login: nmi interrupt received
Stopped at      _Debugger+0x6:  unlk    a6
db> bt
No such command
db> ?
Bad character
?
db> help
machine     print       examine     x           search      set         write
w           delete      d           break       dwatch      watch       step
s           continue    c           until       next        match       trace
call        ps          callout     show
db> tracfe
No such command
db> trace
_Debugger(e080214,ee5bfd8,e08031a,0,0) + 6
_nmi_intr(0,0,2e87e,dffe9e0,dffe994) + 22
_isr_autovec(7c) + 68
__isr_autovec() + a
db> 

  At this point, if I continue, then the machine freezes, requiring a
cold boot. (power switch... BREAK on ttya doesn't cut it)

  Ideas? What is NMI hooked up to?