Great links explaining tests by microsoft, browser comparisons and good suggestions to find the problem
Microsoft Benchmark Testing 2008
The best suggestion detailed above, is to download the Video then play it back.
If there are no problems this means that it is not your computer at fault, but your Broadband or Wireless Network Connection.
Use an Ethernet/Network (RJ45 Cable) connected direct to your router and stream a video this will then eliminate whether it is the wireless connection
