When trying to determine why a cast is stuttering, there are 2 categories to look at, hardware and network:
Hardware includes their own computer and the current state of the jtv network.
Network is their local intranet (AKA inside your home network), the real connection they get from their isp (gotten from a speedtest.net test), the max amount the jtv proxy server will give them, and the network connections between their home and said proxy server.
As you can see, there are a lot of variables, some of which the you can't actually check. But the important thing to be learned from this topic is just because a speedtest says you can upload at 80 mb/s like I can
does not mean you are actually going to be able to stream at the bitrate to Justin/Twitch.tv.
A general rule of thumb is keep it below 3000kbps, never have it above 4000kbps on ANY of their proxy servers (I've run tests on each one). Also remember is if you get 3000 kbps on a server at 4 am in the morning on a Sunday (when no one else is broadcasting) does not mean you will also be able to get the same bitrate at 9pm on a Thursday or Friday night (High Peak times).
If you have any questions or comments, feel free to leave them here.
Update: If you are using XSplit, these are the servers you see: