Sometimes, I suspect it is the video itself. I have just watched three You Tube videos. Two streamed fine, but the third kept stopping. If it had been my computer (which is new) or my Internet connection, they all should have behaved in the same way, so my conclusion is that it was the video itself. Maybe the original that was uploaded was in a less compressed form (Most video formats seem to involve some degree of compression to keep the file sizes reasonable).