Update your flash player (http://get.adobe.com/flashplayer/?promoid=BUIGP
). Update Chrome (http://www.google.com/chrome/intl/en/make/download.html?brand=CHKZ
Update your video card drivers. Easiest way to do this assuming you're using Windows 7:
Click on the Windows button.
RIGHT click on computer.
Select 'Manage' from the popup list. (should have a little shield logo by it)
In the right side box, click 'Device Manager'
Click on 'Display Adapters'
Right click and select 'Update Driver Software'
In the window that pops up, select the 'search automatically for updated driver software'.
Searching on Google shows that quite a few people are having issues like this, and different things are working for different people.
The WAY easier solution? Use a different browser for watching Youtube.