I use Firefox for youtube on my cube because the recently released official Amazon Youtube app frequently locks up the user interface so that only the Home button responds. Many others are experiencing this and note it is NOT a buffering issue.
Fortunately (for me at least) Youtube via Firefox works perfectly. Have you tried the steps suggested in post #2 above? The most obvious cause of buffering, of course, is a poor internet connection. Can you run HD 1080p for Amazon Prime videos without buffering? (Click "down" on the remote ring while playing a video to see this. You should see "HD 1080p" at lower left. Allow the video to play for 30 sec before doing this.)