Ok,
Check that in the Mac Firewall (System Preferences > Security > Firewall that the Advanced Button does not show you doing Stealth ?
In the main part of the Firewall make sure you are using either Allow All or Allow Specifics and that iChat is then in the list.
iChat is not an Essential)
Are you changing from Ethernet to WiFi when iChat is Open ?
If so do the change before staring iChat.
Also try Rebooting the Wifi Device (Router) by turning the power off for 10 secs.
If I asked would you know how to change the Channel the WiFi Device uses ?
The Mac will see any of the channels that can be used so only the device would need changing.
If so what channel is it using now ?
8:39 PM Thursday; January 22, 2009