BTW, it just happened again, only before that there had been
some console messages:

sd0(ahc0:0:0): Queue Full
sd0(ahc0:0:0): Queue Full
ahc0:A:0: ahc_intr - referenced scb not valid during seqint 0x71 scb(13)
ahc0: WARNING no command for scb 13 (cmdcmplt)
QOUTCNT == 0
ahc0:A:0: ahc_intr - referenced scb not valid during seqint 0x71 scb(15)
ahc0: WARNING no command for scb 15 (cmdcmplt)
QOUTCNT == 0

Two things of interest: #1, I cannot break into DDB when this happens,
although I can exit X and kill running processes; #2, I exited a running
netscape with ALT+Q and it caused the disk to do audible things, just
as normal.

Sadly, it seems time to revert to a non-tag-queueing kernel.
(But I'm willing to test any fixes!)

	Sean.