almost certainly your internet connection is not fast enough to view the videos so they are not downloading quick enough to keep up with you viewing them.
Some sites (such as youtube) will download the video in full so the second time round it doesn't have to be downloaded again so you can watch it smoothly, other sites (especially live TV sites or places like BBC Iplayer) don't store the video after the first watch so even if you play it again it has to be downloaded all over from scratch.
All you can do is see if you can get a faster connection.