conflicting interfaces: martians sighted off the starboard bow!

For the purpose of online gaming, my firewall has two connections to
my cable isp. One for the firewall itself (which also runs a webserver
and dns) and one for my more powerful gaming machine within the
network.

Now the problem: the two interfaces to my cable ISP, eth0 and eth3,
cannot be enabled at the same time without a massive stream of martian
reports going to the syslog. A typical example:

Nov 24 02:04:42 darkbox kernel: NET: 87 messages suppressed.
Nov 24 02:04:42 darkbox kernel: martian source 12.250.204.79 from
12.250.204.1, on dev eth3
Nov 24 02:04:42 darkbox kernel: ll header:
ff:ff:ff:ff:ff:ff:00:01:5c:22:62:42:08:06
Nov 24 02:04:42 darkbox kernel: Packet log: input DENY eth3 PROTO=17
10.163.224.1:67 255.255.255.255:68 L=328 S=0x00 I=13533 F=0x0000 T=64
(#92)
Nov 24 02:04:47 darkbox kernel: NET: 75 messages suppressed.
Nov 24 02:04:47 darkbox kernel: martian source 12.211.175.108 from
12.211.168.1, on dev eth3
Nov 24 02:04:47 darkbox kernel: ll header:
ff:ff:ff:ff:ff:ff:00:01:5c:22:62:42:08:06
Nov 24 02:04:52 darkbox kernel: Packet log: input DENY eth3 PROTO=17
10.163.224.1:67 255.255.255.255:68 L=328 S=0x00 I=13651 F=0x0000 T=64
(#92)
Nov 24 02:04:52 darkbox kernel: NET: 82 messages suppressed.
Nov 24 02:04:52 darkbox kernel: martian source 67.173.40.40 from
67.173.40.1, on dev eth0
Nov 24 02:04:52 darkbox kernel: ll header:
ff:ff:ff:ff:ff:ff:00:01:5c:22:62:42:08:06
Nov 24 02:04:57 darkbox kernel: NET: 100 messages suppressed.

The output from ifconfig is as follows:
[root@darkbox dudemaster]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:90:27:3F:C2:CF
inet addr:12.211.168.166 Bcast:255.255.255.255
Mask:255.255.248.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:90039 errors:0 dropped:0 overruns:0 frame:0
TX packets:1918 errors:0 dropped:0 overruns:7 carrier:0
collisions:0 txqueuelen:100
RX bytes:6820784 (6.5 Mb) TX bytes:341372 (333.3 Kb)
Interrupt:18 Base address:0xe000

[root@darkbox dudemaster]# ifconfig eth3
eth3 Link encap:Ethernet HWaddr 00:06:25:3A:B4:51
inet addr:67.173.42.213 Bcast:255.255.255.255
Mask:255.255.252.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:39246 errors:0 dropped:0 overruns:0 frame:0
TX packets:11 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:2404639 (2.2 Mb) TX bytes:3462 (3.3 Kb)

eth0 is a standard ethernet connection to my cable modem, and eth3 is
a usb connection (using the CDCEther module) to the cable modem. Both
connections work fine by themselves, but don't work at all when both
are enabled.

Any ideas how to fix this? - I'm stumped.

Thanks,
icy.