Problems with NAT after updating from 1.6 kernel to 4.0

I have been using IPNat for approximately 5 years unmodified.  Updating 
to new x86 hardware and the new 4.0 GENERIC kernel (but keeping mostly 
the same userspace), my ipnat gateway now has a very annoying 3 second 
delay added to the establishment of every outgoing TCP connection.

I've traced the 3 second delay with tcpdump and found the culprit: the 
first packet on a newly established TCP connection is always dropped 
(after the 3 way handshake).  It is received okay on the first interface 
but never transmitted to the second interface.  3 seconds seems to be 
the retransmit timeout on my Windows PC on the firewalled side.  After 
that first packet drop and 3 second retransmit delay, the rest of the 
TCP session is fine.

Anybody have any ideas?  The 2 ethernet devices are sip0 and bge0 and 
this is my ipnat.conf:

map sip0 from 192.168.0.0/16 ! to 192.168.0.0/16 -> 67.40.67.44/32 proxy 
port ftp ftp/tcp
map sip0 from 192.168.0.0/16 ! to 192.168.0.0/16 -> 67.40.67.44/32 
portmap tcp/udp 40000:60000
map sip0 from 192.168.0.0/16 ! to 192.168.0.0/16 -> 67.40.67.44/32



//Jesse Off