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Try to pause the video so it finishes fully downloading before playing.
The problems I had with a 1.0Mbit internet connection largely went away when I upgraded to a 3.0Mbit connection.
Popular 'viral' videos could be viewed by so many people at once, that the servers have a hard time keeping up with demand.
And, sometimes you just need a faster computer or a faster internet connection, especially if you're trying to watch in full-screen or HD on an older, slower computer. My mid-2007 MacBook has this problem occasionally, especially with content produced with Adobe Flash.