To ensure that High Quality videos from YouTube always play in the highest quality available, simply follow these steps:
From any page on YouTube click the "Account" link.
On the following page select "Playback Setup"
On the "Playback Setup" page select "I have a fast connection, always play higher-quality video when it's available.
After you are finished with these simple steps, you are ensured to receive your video content in the highest quality available.