Frequent lockups with 5.2-BETA

Since upgrading from 5.1-RELEASE to 5.2-BETA, I've been
experiencing hard lockups once or twice every day.

I managed to get a trace by enabling the watchdog, which
put me into the debugger. This is the trace:

db> trace
Debugger(c06f85ec,1bc169d,0,c0754fc4,c0754bf8) at Debugger+0x54
watchdog_fire(d73e2bcc,c067b433,c0c57100,0,d73e2bcc) at watchdog_fire+0xc1
hardclock(d73e2bcc,0,0,d73e2b98,c43bd400) at hardclock+0x15a
clkintr(d73e2bcc,d73e2b9c,c06c6bf6,0,c1925000) at clkintr+0xe9
intr_execute_handlers(c0756be0,d73e2bcc,c0c570a0,1000,c0c61300) at intr_execute8
atpic_handle_intr(0) at atpic_handle_intr+0xef
Xatpic_intr0() at Xatpic_intr0+0x1e
--- interrupt, eip = 0xc068db74, esp = 0xd73e2c10, ebp = 0xd73e2c14 ---
uma_zone_slab(c0c61300,1,0,c068e516,c074edd8) at uma_zone_slab+0x4
uma_zalloc_internal(c0c61300,0,1,0,d73e2c80) at uma_zalloc_internal+0x5c
bucket_alloc(2f,1,0,0,0) at bucket_alloc+0x65
uma_zfree_arg(c0c48240,c49121b8,0,c1922580,3600) at uma_zfree_arg+0x2c6
tcp_hc_purge(0,c1922580,161e9,142b64fd,c05d7c70) at tcp_hc_purge+0x11f
softclock(0,0,0,0,c192b54c) at softclock+0x25e
ithread_loop(c1922580,d73e2d48,0,11,55ff44fd) at ithread_loop+0x1d8
fork_exit(c0524ec0,c1922580,d73e2d48) at fork_exit+0x80
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xd73e2d7c, ebp = 0 ---

This is the dmesg from the last boot:

Copyright (c) 1992-2003 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 5.2-BETA #7: Wed Nov 26 17:24:32 CET 2003
    root_at_borg.borderworlds.dk:/usr/obj/usr/src/sys/BORG
Preloaded elf kernel "/boot/kernel/kernel" at 0xc0823000.
Timecounter "i8254" frequency 1193182 Hz quality 0
CPU: Intel(R) Celeron(R) CPU 1.70GHz (1716.04-MHz 686-class CPU)
  Origin = "GenuineIntel"  Id = 0xf13  Stepping = 3
  Features=0x3febfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM>
real memory  = 536805376 (511 MB)
avail memory = 511885312 (488 MB)
Pentium Pro MTRR support enabled
acpi0: <AMIINT INTEL845> on motherboard
pcibios: BIOS version 2.10
Using $PIR table, 15 entries at 0xc00f7810
acpi0: Power Button (fixed)
Timecounter "ACPI-fast" frequency 3579545 Hz quality 1000
acpi_timer0: <24-bit timer at 3.579545MHz> port 0x808-0x80b on acpi0
acpi_cpu0: <CPU> port 0x530-0x537 on acpi0
acpi_cpu1: <CPU> port 0x530-0x537 on acpi0
device_probe_and_attach: acpi_cpu1 attach returned 6
acpi_button0: <Power Button> on acpi0
pcib0: <ACPI Host-PCI bridge> port 0xcf8-0xcff on acpi0
pci0: <ACPI PCI bus> on pcib0
agp0: <Intel Generic host to PCI bridge> mem 0xe0000000-0xe3ffffff at device 0.0 on pci0
pcib1: <PCI-PCI bridge> at device 1.0 on pci0
pci1: <PCI bus> on pcib1
pcib0: slot 1 INTA is routed to irq 5
pcib1: slot 0 INTA is routed to irq 5
pci1: <display, VGA> at device 0.0 (no driver attached)
pcib2: <ACPI PCI-PCI bridge> at device 30.0 on pci0
pci2: <ACPI PCI bus> on pcib2
pcib2: slot 10 INTA is routed to irq 10
pcib2: slot 13 INTA is routed to irq 3
pcm0: <CMedia CMI8738> port 0xdc00-0xdcff irq 10 at device 10.0 on pci2
em0: <Intel(R) PRO/1000 Network Connection, Version - 1.7.19> port 0xd800-0xd83f mem 0xdfec0000-0xdfedffff,0xdfee0000-0xdfefffff irq 3 at device 13.0 on pci2
em0:  Speed:N/A  Duplex:N/A
isab0: <PCI-ISA bridge> at device 31.0 on pci0
isa0: <ISA bus> on isab0
atapci0: <Intel ICH4 UDMA100 controller> port 0xfc00-0xfc0f,0-0x3,0-0x7,0-0x3,0-0x7 at device 31.1 on pci0
ata0: at 0x1f0 irq 14 on atapci0
ata0: [MPSAFE]
ata1: at 0x170 irq 15 on atapci0
ata1: [MPSAFE]
pci0: <serial bus, SMBus> at device 31.3 (no driver attached)
atkbdc0: <Keyboard controller (i8042)> port 0x64,0x60 irq 1 on acpi0
atkbd0: <AT Keyboard> flags 0x1 irq 1 on atkbdc0
kbd0 at atkbd0
psm0: <PS/2 Mouse> irq 12 on atkbdc0
psm0: model MouseMan+, device ID 0
fdc0: cmd 3 failed at out byte 1 of 3
sio0 port 0x3f8-0x3ff irq 4 on acpi0
sio0: type 16550A, console
ppc0 port 0x778-0x77b,0x378-0x37f irq 7 drq 3 on acpi0
ppc0: SMC-like chipset (ECP/EPP/PS2/NIBBLE) in COMPATIBLE mode
ppc0: FIFO with 16/16/9 bytes threshold
ppbus0: <Parallel port bus> on ppc0
plip0: <PLIP network interface> on ppbus0
lpt0: <Printer> on ppbus0
lpt0: Interrupt-driven port
ppi0: <Parallel I/O> on ppbus0
acpi_cpu1: <CPU> port 0x530-0x537 on acpi0
device_probe_and_attach: acpi_cpu1 attach returned 6
fdc0: cmd 3 failed at out byte 1 of 3
npx0: [FAST]
npx0: <math processor> on motherboard
npx0: INT 16 interface
orm0: <Option ROM> at iomem 0xe0000-0xe0fff on isa0
pmtimer0 on isa0
fdc0: cannot reserve I/O port range (6 ports)
sc0: <System console> at flags 0x100 on isa0
sc0: VGA <16 virtual consoles, flags=0x100>
sio1: configured irq 3 not in bitmap of probed irqs 0
sio1: port may not be enabled
vga0: <Generic ISA VGA> at port 0x3c0-0x3df iomem 0xa0000-0xbffff on isa0
Timecounter "TSC" frequency 1716042336 Hz quality 800
Timecounters tick every 10.000 msec
IPv6 packet filtering initialized, default to accept, logging limited to 100 packets/entry
ipfw2 initialized, divert disabled, rule-based forwarding enabled, default to accept, logging limited to 100 packets/entry by default
acpi_cpu: throttling enabled, 8 steps (100% to 12.5%), currently 100.0%
GEOM: create disk ad0 dp=0xc4508960
ad0: 76319MB <ST380011A> [155061/16/63] at ata0-master UDMA100
acd0-5: CDROM with 6 CD changer <CD-C68E> at ata1-master PIO4
acd6: DVDROM <CREATIVEDVD5240E-1> at ata1-slave PIO4
acd0-5: FAILURE - LOAD_UNLOAD status=51<READY,DSC,ERROR> sensekey=ILLEGAL REQUEST error=4<ABORTED>
acd0-5: FAILURE - LOAD_UNLOAD status=51<READY,DSC,ERROR> sensekey=ILLEGAL REQUEST error=4<ABORTED>
acd0-5: FAILURE - LOAD_UNLOAD status=51<READY,DSC,ERROR> sensekey=ILLEGAL REQUEST error=4<ABORTED>
acd0-5: FAILURE - LOAD_UNLOAD status=51<READY,DSC,ERROR> sensekey=ILLEGAL REQUEST error=4<ABORTED>
acd0-5: FAILURE - LOAD_UNLOAD status=51<READY,DSC,ERROR> sensekey=ILLEGAL REQUEST error=4<ABORTED>
Mounting root from ufs:/dev/ad0s3a
WARNING: / was not properly dismounted
WARNING: /tmp was not properly dismounted
WARNING: /usr was not properly dismounted
/usr: mount pending error: blocks 8 files 1
/usr: superblock summary recomputed
WARNING: /var was not properly dismounted
/var: mount pending error: blocks 8 files 1
/var: superblock summary recomputed
em0: Link is up 100 Mbps Full Duplex

If you need more information, I'll gladly try to make it available.

Thanks.

-- 
Best regards
    Christian Laursen