This is my route -n Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 213.250.143.240 0.0.0.0 255.255.255.240 U 0 0 0 eth0 172.16.4.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1 172.16.16.0 0.0.0.0 255.255.255.0 U 0 0 0 eth3 172.16.8.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2 0.0.0.0 172.16.16.254 0.0.0.0 UG 0 0 0 eth3
an easy try: what is the default gateway setting on the internal networks hosts?
for instance, on network 172.16.4.0/24, if the router nic is on 172.16.4.1, the default gateway on any other host must be 172.16.4.1 (hope i've been clear :-) )
eventually, you can try to dump completely static routing. static routing is good for causing headaches.
hope this helps. pietro.