This may happen sometimes when you have too many plugins, a bad browser, or you have HD turned ON and the compatibility is not working properly on your device.
The first step is to turn HD OFF in your video and see if this solves the problem.
Another option is to go into a clean browser window. If you are using Google Chrome you can open up your browser in Incognito window (which means a private browsing session). This often solves the problem. To go into incognito you can:
- In the top-right corner of the browser window, click the Chrome menu.
- Select New Incognito Window.
- You can also press Ctrl+Shift+N (Windows, Linux, and Chrome OS) and ⌘-Shift-N (Mac).