em driver problems? taskq em goes up to 100% CPU

[ibmed]

Sorry - didn't know where to post my question to: here or freebsd-net:

The problem is as follows:
I have two FreeBSD boxes that do ipfw nat.
I got two Intel cards (Intel <EXPI9404PT> PRO/1000 PT Quad Port (OEM) PCI-E
x4 10/100/1000Mbps) - one for each box, since the msk cards that boxes had
onboard had problems like "kernel: msk0: Rx FIFO overrun!". em cards solved
the FIFO problem, BUT: there's now a problem with spontaneous peaks of
system load: irrelatively to anything I can guess the system processes taskq
em0/em2 begin to eat 100% of CPU (and stay like that for about 1 or 2
minutes). And when that happens, traffic stops to pass through the box. The
problem is there on both boxes.

I tried upgrading both of them to RELENG_7_0, RELENG_7_1 (RC2 currently),
RELENG_7 - the problem remains.

Any suggestions on what can I do to solve the problem?
I have just no idea on what causes that and what to do to fix it. I would
really appreciate your help..

And just to be thorough, here's my kernel options:
options IPFIREWALL
options IPFIREWALL_VERBOSE
options IPFIREWALL_VERBOSE_LIMIT=400
options IPFIREWALL_DEFAULT_TO_ACCEPT
options IPFIREWALL_FORWARD
options IPFIREWALL_NAT
options LIBALIAS
options DUMMYNET
options IPDIVERT

fastforwarding is on, polling is off:
net.inet.ip.fastforwarding: 1

em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=19b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING, VLAN_HWCSUM,TSO4>
ether 00:00:10:01:1:01
inet 10.1.1.1 netmask 0xffffff00 broadcast 10.1.1.255
media: Ethernet autoselect (1000baseTX <full-duplex>)
status: active 
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