Click or tap the 'cog' icon then the bar graph icon to select a lower video resolution (eg 480p or 240p).
Make sure you have either the latest version of Flash running on your computer, or use a browser with native HTML5 support such as Chrome or Safari.
Update your browser to the latest version.
Restart your computer/device.
Check your internet connection - if other videos (eg YouTube) are also choppy, you might need to speak with your IT technician or internet service provider.