FreeBSD's ng_nat stopping pass the packets periodically

I have FreeBSD router:

#uname
9.1-STABLE FreeBSD 9.1-STABLE #0: Fri Jan 18 16:20:47 YEKT 2013

It's a powerful computer with a lot of memory

#top -S
last pid: 45076;  load averages:  1.54,  1.46,  1.29                                      up 0+21:13:28  19:23:46
84 processes:  2 running, 81 sleeping, 1 waiting
CPU:  3.1% user,  0.0% nice, 32.1% system,  5.3% interrupt, 59.5% idle
Mem: 390M Active, 1441M Inact, 785M Wired, 799M Buf, 5008M Free
Swap: 8192M Total, 8192M Free

  PID USERNAME    THR PRI NICE   SIZE    RES STATE   C   TIME   WCPU COMMAND
   11 root          4 155 ki31     0K    64K RUN     3  71.4H 254.83% idle
   13 root          4 -16    -     0K    64K sleep   0 101:52 103.03% ng_queue
    0 root         14 -92    0     0K   224K -       2 229:44 16.55% kernel
   12 root         17 -84    -     0K   272K WAIT    0 213:32 15.67% intr
40228 root          1  22    0 51060K 25084K select  0  20:27  1.66% snmpd
15052 root          1  52    0   104M 22204K select  2   4:36  0.98% mpd5
   19 root          1  16    -     0K    16K syncer  1   0:48  0.20% syncer

Its tasks are: NAT via ng_nat and PPPoE server via mpd5.

Traffic through - about 300Mbit/s, about 40kpps at peak. Pppoe sessions created - 350 max.

ng_nat is configured by by the script:

 /usr/sbin/ngctl -f- <<-EOF                                            

             mkpeer ipfw: nat %s out                                                                               
             name ipfw:%s %s                                                                                       
             connect ipfw: %s: %s in                                                                               
             msg %s: setaliasaddr 1.1.%s

There are 20 such ng_nat nodes, with about 150 clients.

Sometimes, the traffic via nat stops. When this happens vmstat reports a lot of FAIL counts

vmstat -z | grep -i netgraph
ITEM                   SIZE  LIMIT     USED     FREE      REQ FAIL SLEEP
NetGraph items:          72,  10266,       1,     376,39178965,   0,   0
NetGraph data items:     72,  10266,       9,   10257,2327948820,2131611,4033

I was tried increase

net.graph.maxdata=10240                                                                                           
net.graph.maxalloc=10240

but this doesn't work.

It's a new problem (1-2 week). The configuration had been working well for about 5 months and no configuration changes were made leading up to the problems starting.

In the last few weeks we have slightly increased traffic (from 270 to 300 mbits) and little more pppoe sessions (300->350).

Help me please, how to find and solve my problem?

Upd: Info about network cards:

# pciconf -lv | grep -B3 network   
em0@pci0:0:25:0:        class=0x020000 card=0x35788086 chip=0x15028086 rev=0x05 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = '82579LM Gigabit Network Connection'
    class      = network
--
em1@pci0:2:0:0: class=0x020000 card=0x35788086 chip=0x10d38086 rev=0x00 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = '82574L Gigabit Network Connection'
    class      = network

UPD: There is 2 "top" output https://gist.github.com/korjavin/9190181

when I swith net.isr.dispatch to hybrid. After this, I have tons of mpd processes (don't know why) and one CPU to 100% of interrupt, and after 10 minutes of work it was rebooted, due to big packet lost.

UPD: Happened again There is "top" output before reboot and after https://gist.github.com/korjavin/9254734

looks like problem in ng_queue proccess, which eating CPU to much. Since my first post, there much more sessions and traffics. About 400 pppoe , and 450Mbit/s