I'm assuming you've already gone into your account settings > playback and chosen "Always choose the best quality for my connection and player size", and conversely, you haven't accidentally checked " I have a slow connection. Never play higher-quality video"
Theoretically those settings should deliver the highest quality your connection would support. Have you tried a different browser like Chrome or Firefox to see if the problem persists there? That way you can whittle it down from a network or system-wide issue to a browser specific one.
Finally, I know I've occassionally been pulling out my hair due to playback issues, only to discover that Safari has somehow auto-enrolled me in the youtube html5 trial, which is incomprehensibly terrible. you can control your enrolment in that at the url:
http://www.youtube.com/html5
those are all my thoughts. good luck!