If video doesn't play smoothly then click the pause button (or anywhere on the video) to pause and wait a bit for video to load. You can see the progress of the video loading on the control bar.
If the webpage has a 'smaller video' button you could try that.
If you can't see some videos on this site you may need a (free): updated flash player. Some of the videos use a flash player with fallback to HTML5 for apple / iOS.
There may be a few Quicktime (H264) videos remaining for which you may need (free): Quicktime 7 or above.
If you're using an older iPhone or iPad etc and can't see the videos, then check for an 'iPhone iPad version' button at the top of page.