Google Chrome - Flash on Youtube issue
Honestly, I think that's just part of Flash on OS X.  You could test the same YouTube videos in other browsers, as Chrome contains its own Flash rather than use the /Library plug-in version, but every YT video I've played on a Mac has always had that buffering pause a few seconds into it.  (Vimeo is usually smooth.)

There have been some sorta-hacks to set Mac browsers to use HTML5 instead of Flash, however: you might start with Jon Gruber's always-excellent blog though that may be somewhat outdated.