Kernel 2.6.38.3 panic and hangs on network traffic

Hi!
I starting up new server for the test. It's task is terminate L2TP&PPTP 
users. And when i have opened server (NAS) for users - its crush quickly :(
What it's could be?
Hardware is Intel Q6600 with 4GB, Intel nic (igb from inel, latest, RSS 
enabled 4 queue per RX,TX on both nic). Software is accel-ppp and 
openl2tpd, ppp redirects to ifb(no ip is up on it, just shaping), ipset, 
iptables.
If need more info - i can give it. Strange is - sometimes (mostly) i see 
panic, but sometime it's just hangs. Only reset helps...
All IF'ses are IPV4. What on the users side - must be ipv4, but i think 
not the only...