I think I am having this issue too, and I am not signed up to the Youtube HTML5 beta.
When the window freezes, it's always just the current window and all tabs in it. Other windows and subsequent new windows are unaffected unless I use them to visit Youtube.
Additionally, weird upside-down text appears at the top of the window, as in the screenshot:
Blank portions of the window are caused by my shrinking and growing the window. The audio of the video (IYSWIM) plays fine while this is going on.