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Maybe a problem with the graphics card?

Do you have other programs open as well as the internet? If so maybe its the RAM...or lack there of.
 
i've got the same macbook as you and it plays fine on mine, slight motion lag when viewing it in standard, pressed the full button and it played perfectly in full screen.
Youtube in HD on my macbook owns, looks soooo good on the glossy screen at full brightness, j'adore ca
 
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