Hi,
The Modem should not reset itself.
I would ask Comcast about that.
Point out that you are limiting the Bandwidth.
They will still try to blame iChat.
Many people who have come here with Comcast problems say limiting the Bandwidth works so it is not iChat.
Comcast are known to Manage their Bandwidth in an effort to Combat Peer-to-Peer File Sharing.
Ask them directly if they distinguish between different Apps that do Peer-to-Peer contact. You may not get a complete of straight answer but your service may improve anyway.
It maybe just a fault with the device.
Also go to the iChat > Preferences > Accounts
In the Account Info tab deselect "Use this Account" to Log out.
In the Server Setting tab change the port from 5190 to 443 (in iChat 5 you have to hit enter to "Set" the port and SSL might have to be On first)
8:31 PM Sunday; January 3, 2010
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"