After some Googling and other searching on the net here are some things it seems that other people suggest checking. Overall it seems Apple have built a rather sensitive piece of software in iChat. It seems very tricky to get it right. For some it just works, for others it doesn't work at all. Anyway, here goes ;-)
Check firewall on routers
These ports should be open on NAT routers (and you can open the same on your Mac):
5060, 5190, 5220, 5222, 5298, 5353, 5678, and 16384 through 16403.
You can also put your Mac on the router's DMZ or try to connect your Mac straight to the modem.
Here's some more explanation about ports:
Quote:
iChat AV uses a range of ports for different purposes. When conferencing, ports 16384 to 16403 are used to send, receive, and optimize AV streams. For a single conference, 4 ports from that range of 20 are utilized to send and receive audio and video. Additionally, port 5060 is used for signaling and initiation of AV chat invitations, and 5678 is used for SNATMAP, for a total of 22 ports open. Ports used for other purposes, such as Bonjour (formerly "Rendezvous") and file transfer, are listed in the next two sections.
Tip: Traffic that goes through ports may be subdivided into different types, including TCP and UDP. iChat uses both of these, but mainly UDP. Advanced users can find more information on this in the Notes section below.
Ports to open for Mac OS X firewall
When using the built-in Mac OS X firewall, you only need to open these ports: 5060, 5190, 5297, 5298, 5678, 16384 through 16403.
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Quicktime settings
Go to System Preferences > Quicktime > Streaming tab
Set the drop down to your Connection speed download speed or to a max of 1.5Mbps
In iChat
In iChat > Preferences > Video section > Bandwidth Limit se the drop down to NONE.
Change iChat port to it to 443 or at least anything but the standard 5190.
Rebooting Mac
Some users say rebooting the Mac right before trying to iChat and not running any other applications at the same time as iChat can help.
Delete preferences
Go to /Users/(your account)/Library/Preferernces and delete com.apple.ichat.AIM.plist
More information
In Apple's discussion groups there are some threads with suggestions that may help:
here,
here,
here, and
here.
Do a speed test
You can do a
test of the speed of your connection. I'm on 2Mbps AlShamil ADSL and I got 2057kbps down and 353kbps up. I think I read somewhere among the pages linked to above that you need 100kbps to do a/v chat with iChat.