skbuf problem

Hello all,

I wonder what make skbuf exhausted - asymetric routing, many syn
packets, traffic flood(many pps) ???? I have two linux firewals.
One(A) of both has skbuf problem. Sometimes there is no skbuf, so I
can see a lot of RX dropped packets and network is down as well.
It's really weird becuase the other machine(B) normally is under heavy
traffic than A machine but it's working properly and there is no issue
like that skbuf is exhausted.
Any idea what caused skbuf exhaust and how to fix it?

cat /proc/net/skb_stat
Limit Remaining Dropped.
40000 0 3290123

Thanks,
Dimitri