Postgresql locks up server - no response at all

FreeBSD 5.2.1-P8 running on dual Xeon supermicro system with vinum data
drive and em network interfaces. I have been having a problem with the
system simply locking up every couple days. No response from the
keyboard, network, nothing. As if it is in some state of IRQ locking. I
see nothing in the messages, even with DDB and DDB_UNATTENDED enabled in
kernel. The system runs 4GB of ram with the following modifications to
kernel:

cpu             I486_CPU
cpu             I586_CPU
cpu             I686_CPU
<snip>
options         SHMMAXPGS=65536         # ********************
options         SEMMNI=40               # added for posgresql
options         SEMMNS=240              # allows for around
options         SEMUME=40               # 180 simultaneous connections
options         SEMMNU=120              # ********************
<snip>
# Debugging for use in -current
options         DDB                     #Enable the kernel debugger
options         DDB_UNATTENDED          #Don't panic on DDB but log it
#options        INVARIANTS              #Enable calls of extra sanity
checking
options         INVARIANT_SUPPORT       #Extra sanity checks of internal
#options        WITNESS                 #Enable checks to detect dead ..
#options        WITNESS_SKIPSPIN        #Don't run witness on spinlocks 
# Deal with kmem issues
options                 VM_KMEM_SIZE_SCALE="4"
options                 VM_KMEM_SIZE_MAX="(512*1024*1024)"
options                 KVA_PAGES=512


/boot/loader.conf:
vinum_load="YES"
vinum.autostart="YES"
#kern.maxdsiz="1073741824"
#kern.dfldsiz="1073741824"

I had experimented in loader.conf with the dsiz settings to no avail,
still get lockups. Got lockups with and without the DDB settings. It
would be helpful if I could see some type of error being generated, but
nothing; the attached terminal has utterly no messages beyond normal
system messages, everything just stops responding.

After the last lockup and reboot, I sysctl machdep.hlt_logical_cpus=1 to
see if that had any effect. Any other recommendations? adaptive_mutexes?
Any ideas on how to actually find out what is happening?

Sven