Youtube stalling on Macbook Pro

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I'm getting fed up of this.
Every time I try to play Youtube videos they stop after a few seconds. It does this even if I set the resolution to the lowest setting. The buffering bar thing at the bottom of the video just stops moving to the right and when the play bar catches up, the video stops.
I have a 20mb+ cable internet connection and website browsing and downloads from everywhere else are as fast as ever.
I have a Macbook Pro and saw somewhere that I should try turning off the Hardware Video Acceleration but the Flash player settings don't give me the option to do this. It worked perfectly before and the Flash thing is updated to the latest version.
Can someone help me fix this?

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Html5 version of YouTube made no difference. Bear in mind I could watch 1080p videos no problem up until a few weeks ago.
I use Chrome and have just watched HD stuff on Vevo with no problems. And Vimeo too.

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Does the same in Safari. Vimeo/ Vevo fine, Youtube stops after a couple of seconds.
Checked everything and it's up to date.

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Just disabled AdBlocker in Chrome to see if it was that, but made no difference.

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Yep. Virgin.