Most importantly, because such streaming video requires a fast internet speed, have you got a GOOD 3 Mbps connection or faster? If not, then choppy video is possible.. Test you bandwidth speed at one of the links below:

I use the Java test at this site:
http://www.dslreports.com/stest

http://www.pcpitstop.com/internet/Bandwidth.asp

Next, make sure all unnecessary background programs are shut down so your computer resources aren't being drained from the video..

Next, you don't mention how much RAM is installed on the machine or on the video card.. Both should fit the system requirements of the site.

Hope this helps.

Grif