Negative ping times with FreeBSD 8.1-RELEASE on older Celeron system

[Brett Glass]

Here's a puzzler.

I just put FreeBSD 8.1 up on an old (but good) 500 MHz Celeron with 
half a gig of RAM. Interfaces are classic xl (3Com) and dc (DEC 
tulip). Works quite nicely except for one quirk: ping times that 
ought to be positive (no more than 200 ms worst case) are coming 
out negative! Can't figure out what might be causing this. dmesg 
output is as follows:

Copyright (c) 1992-2010 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
         The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 8.1-RELEASE-p2 #5: Fri Apr 15 16:10:53 MST 2011
     brett at washington.lariat.net:/usr/obj/usr/src/sys/WASHINGTON i386
Timecounter "i8254" frequency 1193182 Hz quality 0
CPU: Pentium II/Pentium II Xeon/Celeron (501.14-MHz 686-class CPU)
   Origin = "GenuineIntel"  Id = 0x665  Family = 6  Model = 6  Stepping = 5
   Features=0x183f9ff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,MMX,FXSR>
real memory  = 536870912 (512 MB)
avail memory = 515813376 (491 MB)
acpi0: <AMIINT AMIINT10> on motherboard
acpi0: [ITHREAD]
acpi0: Power Button (fixed)
Timecounter "ACPI-safe" frequency 3579545 Hz quality 850
acpi_timer0: <24-bit timer at 3.579545MHz> port 0x408-0x40b on acpi0
cpu0: <ACPI CPU> on acpi0
pcib0: <ACPI Host-PCI bridge> port 0xcf8-0xcff on acpi0
pci0: <ACPI PCI bus> on pcib0
atapci0: <SiS 620 UDMA66 controller> port 
0x1f0-0x1f7,0x3f6,0x170-0x177,0x376,0xffa0-0xffaf at devic
e 0.1 on pci0
ata0: <ATA channel 0> on atapci0
ata0: [ITHREAD]
ata1: <ATA channel 1> on atapci0
ata1: [ITHREAD]
isab0: <PCI-ISA bridge> at device 1.0 on pci0
isa0: <ISA bus> on isab0
pci0: <unknown> at device 1.1 (no driver attached)
pci0: <serial bus, USB> at device 1.2 (no driver attached)
pcib1: <PCI-PCI bridge> at device 2.0 on pci0
pci1: <PCI bus> on pcib1
vgapci0: <VGA-compatible display> port 0xbc00-0xbc7f mem 
0xee800000-0xeeffffff,0xef6f0000-0xef6fffff
  irq 11 at device 0.0 on pci1
xl0: <3Com 3c905C-TX Fast Etherlink XL> port 0xdc00-0xdc7f mem 
0xefffaf80-0xefffafff irq 11 at devic
e 8.0 on pci0
miibus0: <MII bus> on xl0
xlphy0: <3c905C 10/100 internal PHY> PHY 24 on miibus0
xlphy0:  10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
xl0: Ethernet address: 00:01:03:be:8b:c1
xl0: [ITHREAD]
dc0: <ADMtek AN985 10/100BaseTX> port 0xd800-0xd8ff mem 
0xefffa800-0xefffabff irq 12 at device 9.0 o
n pci0
miibus1: <MII bus> on dc0
ukphy0: <Generic IEEE 802.3u media interface> PHY 1 on miibus1
ukphy0:  10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
dc0: Ethernet address: 00:14:bf:5b:f5:ed
dc0: [ITHREAD]
xl1: <3Com 3c905B-TX Fast Etherlink XL> port 0xd400-0xd47f mem 
0xefffaf00-0xefffaf7f irq 9 at device
  10.0 on pci0
miibus2: <MII bus> on xl1
xlphy1: <3Com internal media interface> PHY 24 on miibus2
xlphy1:  10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
xl1: Ethernet address: 00:40:ca:97:13:7a
xl1: [ITHREAD]
acpi_button0: <Power Button> on acpi0
acpi_button0: enable wake failed
atrtc0: <AT realtime clock> port 0x70-0x71 irq 8 on acpi0
orm0: <ISA Option ROMs> at iomem 
0xc0000-0xc7fff,0xc8000-0xc87ff,0xc8800-0xd7fff pnpid ORM0000 on is
a0
sc0: <System console> at flags 0x100 on isa0
sc0: VGA <16 virtual consoles, flags=0x300>
vga0: <Generic ISA VGA> at port 0x3c0-0x3df iomem 0xa0000-0xbffff on isa0
atkbdc0: <Keyboard controller (i8042)> at port 0x60,0x64 on isa0
atkbd0: <AT Keyboard> irq 1 on atkbdc0
kbd0 at atkbd0
atkbd0: [GIANT-LOCKED]
atkbd0: [ITHREAD]
Timecounter "TSC" frequency 501141912 Hz quality 800
Timecounters tick every 1.000 msec
ipfw2 initialized, divert loadable, nat enabled, rule-based 
forwarding enabled, default to accept, l
ogging disabled
load_dn_sched dn_sched PRIO loaded
load_dn_sched dn_sched QFQ loaded
load_dn_sched dn_sched RR loaded
load_dn_sched dn_sched WF2Q+ loaded
load_dn_sched dn_sched FIFO loaded
ad0: 9787MB <QUANTUM FIREBALLlct10 10 A03.0900> at ata0-master UDMA66
Trying to mount root from ufs:/dev/ad0s1a
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
Bump sched buckets to 64 (was 0)
xl0: promiscuous mode enabled
xl0: promiscuous mode disabled
dc0: TX underrun -- increasing TX threshold
dc0: TX underrun -- increasing TX threshold

Any hints here as to what's wrong?

--Brett Glass