Why does youtube stop playing and then starts again while a circular motion appears in the middle of the screen?

It is very simple youtube uses a streaming system based in flash, this means that the youtube player calls a file of an indetermined size stored in a server, did you ever noticed that in the playing bar below the video there is a red bar that fills the bar progresively this is the bar that shows the amount of video downoading of the file in the server, when the size of the file is to high it takes more time for the video player to download it, what happens is that the playing time is shorter than the download time so the player reaches the amount of video played faster than the video downloaded and it stops until the download progress a littlle bit more to continue playing the video, so if the video has a smaller file size the less the player stops, but small size videos could mean a down of quality in the image of the video depending also in the length of the video and in your internet connection speed.

Good luck