A couple of tips that may help with your issue.
Clear cache and cookies on your browser
Try a different browser- if using Internet Explorer it might not be compatible for viewing these videos.
When on a video or slide it's important not to click into another window or program as that may cause it to freeze.
If you are still experiencing the same issue after these tips please stay on and chat with an advisor and let us know what browser and version you are using. You can find this by clicking on this site What browser? My browser? Is my browser out of date? (whatismybrowser.com)