This happens because the video cannot be downloaded quickly enough from the internet for you to watch it in real time.
How can you cure it ?
If this only happens now again and again, then you may have to live with it. It may be happening when the interent, or the server the video comes from, is busy.
If you have the option, chose a lower quality image.
Or get a faster broadband connection.
If you are connecting to the interent via a wireless router try to connect to the router via a wire.
Getting a better or faster computer will NOT solve this problem as it is the broadband connection that is causng the problem not your computer.