I've got this problem (or something similar) and more.

I downloaded the GENERIC-20 kernel (1.5Q) and rebooted. I ran bonnie 
(over ssh, this is a headless server), but oddly the server went down 
within a few minutes (before bonnie finished). Well, NAT kept 
working, but everything else was down.

I rebooted with a monitor, and ran bonnie (over ssh, ADB didn't seem 
to work, but maybe that's because I didn't plug in the keyboard until 
after it was booted). Within a few minutes, I got a bunch of errors 
on the monitor. (That would be... console?)

Unfortunately, there was no paper handy, and the computers in the 
room were not conveniently situated, so I only got a little bit. (If 
you want more I can do it again, but with paper handy, or maybe over 
serial.) Oddly enough, I got errors for both sd1 and sd0, sd0 having 
/usr on it, but not being tested with bonnie.

esp0: !TC on DATA XFER [intr 10, stat 83, step 4] prevphase 0 resid 2bf0
sd1(esp0:2:0): esp0: timed out [ecb 0x5330d0 (flags 0x1, dleft 2bf0, 
stat 0)], <state 5, nexus 0x5330d0, phase(l 3, c 3, p 3), resid 0, 
msg....
a few more of these...
.
.
In here somewhere it restarts the SCSI bus or something?
.
.

Then lots of sense errors, and others.

Also, there were some errors on boot, according to the logs:

[ using 423378 bytes of netbsd a.out symbol table ]
Copyright (c) 1996, 1997, 1998, 1999, 2000
     The NetBSD Foundation, Inc. All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
     The Regents of the University of California.  All rights reserved.
Jan  6 14:11:55 mwdesign /netbsd:
NetBSD 1.5Q (GENERIC) #20: Mon Jan 1 07:46:56 PST 2001
root2@c610:/usr/cvs/src/sys/arch/mac68k/compile/GENERIC
Apple Macintosh Quadra 650  (68040)
cpu: delay factor 1067
total memory = 20480 KB
avail memory = 15212 KB
using 281 buffers containing 1124 KB of memory
mrg: 'Quadra/Centris ROMs' ROM glue, tracing off, debug off, silent traps
mrg: I/O map kludge for ROMs that use hardware addresses directly.
mainbus0 (root)
obio0 at mainbus0
esp0 at obio0 (quick): address 0x898000: NCR53C96, 16MHz, SCSI ID 7
scsibus0 at esp0: 8 targets, 8 luns per target
adb0 at obio0
asc0 at obio0: Apple Sound Chip
intvid0 at obio0 @ f9001000: DAFB video subsystem, monitor sense 1
intvid0: 640 x 480, monochrome
macfb0 at intvid0
wsdisplay0 at macfb0: console (std, vt100 emulation)
sn0 at obio0: integrated Ethernet adapter
sn0: Ethernet address 08:00:07:76:0c:da
iwm0 at obio0: Apple GCR floppy disk controller
iwm: Chip revision not supported (-77) <----
zsc0 at obio0 chip type 0
zsc0 channel 0: d_speed   9600 DCD clk 0 CTS clk 0
zstty0 at zsc0 channel 0
zsc0 channel 1: d_speed   9600 DCD clk 0 CTS clk 0
zstty1 at zsc0 channel 1
nubus0 at mainbus0
ae0 at nubus0 slot c: Farallon EtherWave Nubus, 32KB memory
ae0: Ethernet address 00:00:c5:3b:06:b4
ae1 at nubus0 slot d: Farallon EtherWave Nubus, 32KB memory
ae1: Ethernet address 00:00:c5:3b:12:de
fpu0 at mainbus0 (mc68040)
ae0: length does not match next packet pointer <----
ae0: len 0000 nlen ff00 start 12 first 00 curr 24 next 00 stop 80 <----
ae0: NIC memory corrupt - invalid packet length 65280 <----
scsibus0: waiting 2 seconds for devices to settle...
esp0: unexpected disconnect; sending REQUEST SENSE <----
probe(esp0:0:0):  Check Condition on CDB: 0x00 00 00 00 00 00 <----
     SENSE KEY:  Hardware Error
      ASC/ASCQ:  Message Error
Jan  6 14:11:56 mwdesign /netbsd:
esp0: target 0: sync negotiation
sd0 at scsibus0 target 0 lun 0: <QUANTUM, LPS270S, 590A> SCSI2 0/direct fixed
sd0(esp0:0:0): max sync rate 4.03MB/s
sd0: 258 MB, 2740 cyl, 2 head, 96 sec, 512 bytes/sect x 528808 sectors
esp0: target 2: sync negotiation
sd1 at scsibus0 target 2 lun 0: <QUANTUM, FIREBALL1080S, 1Q09> SCSI2 
0/direct fixed
sd1(esp0:2:0): max sync rate 4.03MB/s
sd1: 1042 MB, 3835 cyl, 4 head, 139 sec, 512 bytes/sect x 2134305 sectors
adb0 (direct, II series): 0 targets
aed0 at adb0 addr 0: ADB Event device
boot device: sd1
root on sd1a dumps on sd1b
PRAM: 0x3a579811, macos_boottime: 0x3a579809.
root file system type: ffs
IP Filter: v3.4.9 initialized. Default = pass all, Logging = disabled

Thanks for all of your help!
Daniel