When watching a video on YouTube, click the pause button and wait 30 seconds or longer, to download more of the video. If that doesn't do the trick, and you're using Firefox, the problem is likely down to the session-restore feature. This saves any open tabs at 10-second intervals, causing a small delay. The more tabs you have open, the more noticeable the delay will be. To solve the problem, type ‘about:config' (without the quotes) in to Firefox's address bar, then type ‘browser.sessionstore.interval' (again without the quotes) in the filter box. Double-click the entry and change the 10000 milliseconds value to 300000. This will increase the time between tab saves.