FreeBSD7.1/i386 crash (fatal trap 12)

[anborn@list.ru]

Hello,
I have problem with FreeBSD 7.1-STABLE/i386.
We have FreeBSD-based cluster of VPN servers (mpd3.18 as PPTP server), and sometimes its fall down.

#dmesg | less
Fatal trap 12: page fault while in kernel mode
cpuid = 1; apic id = 01
fault virtual address   = 0x1ac
fault code              = supervisor write, page not present
instruction pointer     = 0x20:0x806d673a
stack pointer           = 0x28:0xe6c6ca68
frame pointer           = 0x28:0xe6c6cacc
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 12 (swi1: net)
trap number             = 12
panic: page fault
cpuid = 1
Uptime: 1d13h20m2s
Cannot dump. No dump device defined.
Automatic reboot in 15 seconds - press a key on the console to abort
Rebooting...
# uname -a
FreeBSD vpn-17 7.1-STABLE FreeBSD 7.1-STABLE #6: Wed Mar 11 12:59:09 YEKT 2009     root at vpn:/usr/src/sys/i386/compile/kconf  i386
# nm -n /usr/local/drbd/vpntest/boot/kernel/kernel | grep 806d67
806d671b t ng_pptpgre_rcvdata_lower

Sometimes:

#dmesg | grep 'instruction pointer'
instruction pointer     = 0x20:0x806ca7e1
# nm -n /usr/local/drbd/vpntest/boot/kernel/kernel | grep 806ca7
806ca740 t ng_iface_ioctl

What can I do with it? I don`t have experience in using gdb or programming on C.