The cause of this is a slow internet connection. When you're watching a video online, it has to buffer. This is where it gathers all the information for the video and starts to display it. When you have a slow internet connection, it buffers X amount, and while playing through X, it's trying to load Y, but it loads X+Y so slowly then it reaches X+Y, and still has to load and play Z. See what I'm saying? So the issue is a slow internet connection. Your solutions are as follows: Get faster internet. Try not watching in HD. Quit out of other programs that use the internet, e.g. any games, or shut down your Xbox or whatever, quit out of instant messengers, and lastly, if all else fails, then when the video starts playing, pause it, and wait for it to load the entire video, and then hit play.