Panic on 5.2.1-p8 Current

I'm running FreeBSD with 2 NFS mounts over tcp, apache 1.3.31, 
openwebmail(read/write on both nfs mounted filesystems, /var/mail 
spooldir and webmail userdir) and proftpd.

# uname -a
FreeBSD 5.2.1-RELEASE-p8 #3: Mon Jun 28 07:18:32 BRT 2004

src-all last updated at: Sat Jun 19 09:28:59 BRT 2004.

The problem occurs only when users access the openwebmail, 
reading/writing data on nfs mounted filesystems.

Here my sysctl.conf:
net.inet.tcp.blackhole=2
net.inet.udp.blackhole=1
vfs.nfs.access_cache_timeout=60
net.inet.ip.check_interface=0
net.inet.tcp.inflight_enable=1
net.inet.tcp.inflight_debug=0
net.isr.enable=1
kern.polling.enable=1
kern.polling.user_frac=50
EOF

... and here part of my kernel config file:
machine         i386
cpu             I686_CPU
makeoptions     DEBUG=-g
makeoptions     NO_MODULES=yes
options         NFSCLIENT
options         NFSSERVER
options         QUOTA
options         COMPAT_FREEBSD4
options         KTRACE
options         SYSVSHM
options         SYSVMSG
options         SYSVSEM
options         _KPOSIX_PRIORITY_SCHEDULING
options         DDB
options         INVARIANTS
options         INVARIANT_SUPPORT
options         HZ=2000
options         NMBCLUSTERS=65535
options         DEVICE_POLLING
EOF

Running on:
Dell P. Edge 1600
Single Xeon 2.4Ghz HTT(disabled)
Ram: 1Gb
3 SCSI Disks

Backtrace with gdb in attached file.