OK, so it might be the Multicast setting in your router. The way mine is setup at home is PC, Etd are connected to a switch which is connected to a wireless router. The wireless router is my default gateway. The wireless router WAN port is connected to my cable modem which connects to the internet. In your setup your cable modem is your WAN connection and router for your internal network.
Somewhere in your cable modem setup is the option to Enable/Disable Multicast Services. By default it is Enabled. Try Disabling that and see if it has an affect on your problem. That sounds like a weird option, to disable Multicast Services, but I think the service it provides is sending and receiving Multicast through the modem to the internet. It may be that you are bogging down your router and internet connection, sending your multicast traffic not just to your Etd but also forwarding to the internet. Your service provider more than likely just drops it, so it isn't really going anywhere. I think you can disable the service and not affect anything you are doing on the internet and it should still work between your PC and the internet.
However, no guarantees. The documentation I can find from Motorola about your modem and configuring it is next to worthless when you get into any of the more advanced configuration options. But, give it a shot and see if it helps. You can always turn it back on if it causes a problem.
