4.9-4.11 receive network stack lockup

[Stephen Clark]

Hi List,

I am benchmarking freebsd 4.9, (also tried 4.11), two systems using 
nttcp 1.47 from ports
thru a 100mb switch with two realtek 10/100 nics.
I have 50 gre tunnels going thru 50 vpn tunnels between the two machines 
as well as
quagga/ospfd with all addresses as neighbors.
Using the following script I occasionally get in a state on the sending 
machine where it can send data but not receive it. I have a console that 
I run tcpdump on and I see packets go out but none come back, where on 
the other machine I see the packets come an a response go back.
Any ideas as to what could be happening?

for ((i=1;i<51;i++ )); do nttcp -T -w48 -n 32768 10.1.1.$((i*2))& done

TIA,
Steve