I need assistance in figuring the following network connectivity
problem.

Configuration Info:
Internet <==>Cable Modem/Router<==>Symantec 300 F/W<==>Dell Switch<==>
Desktop PCs/Server (Network 1)

Also connected directly to the Cable Modem/Router is a DLink Wireless
Router which is accessed by another independent group of computer
users, all wireless users. (Network 2) (Network 2 is a new addition as
of today)

The Ambit Cable Modem/Router from Charter has one cable connection port
(for inbound cable) and four Ethernet ports in the back. The DLink
Wireless Router and the Symantec firewall appliance are connected to
two of the four Ethernet ports.

Cable Modem service is from Charter with a static IP.

Network 1 (desktops) is an internal network with IP in the
192.168.0.xxx range and an internal gateway of 192.168.0.4 with DHCP
services provided by the Symantec Firewall appliance (192.168.0.4)

Network 2 (wireless) is an internal network with IP in the
192.168.1.xxx range and internal gateway of 192.168.1.4 with DHCP
services provided by the DLink Wireless router (192.168.1.4)

Network 1 does not need to see anyone on Network 2 nor access any
resources on Network 2. The reverse is also true.


Problem:
Internet connectivity is sporadic to rare on both networks when they
are both connected to the cable modem. If the D-Link router (and thus
the entire Network 2) is disconnected from the cable modem, the desktop
network works fine. If the Symantec firewall is disconnected from the
Cable modem (and thus the entire Network 1), the wireless network is
better. When both are connected, internet connection for the wireless
users as well as the desktop users is limited to rare. Interestingly, I
can ping the Static IP address provided by Charter from both internal
networks without any lost packets when both networks are connected to
the Cable modem. I can also connect to internal interface for the DLink
router and the Symantec firewall without any problems. I can ping the
internal gateway addresses on each network without any problems. I can
also tracert to the static IP from both networks when they are both
connected to the Cable modem (although tracert is slow and has a few to
many timeout requests).

I would appreciate any suggestions/ideas on what might be the cause of
poor connectivity and how it can be fixed. I am wondering if the Cable
modem is performing some type of routing between the two networks as
evidenced by the slow tracert/timeouts. I will add that Charter tech
support remotely connected to the modem and advised that no routing is
being performed on the Cable modem..

Thanks in advance for the help.

Cheers
Subbarayudu