It occurs most frequently when I switch to full-screen mode, but also occurs when the videos are normal-sized. When I reach that point during playback, I have to reload the page and start over from the place where it gave up. The videos will load for a while, then stop at some point.
I have also tried other web sites besides YouTube. I have plugged directly into the modems (so as to bypass wireless slowness), tried at all hours of the day (to avoid congestion problems), and used all of my web browsers. I have used the computer in both Ohio and Maine in the past week, through completely different (cable) ISPs. I currently have Chrome, Firefox, Internet Explorer, and Opera installed. It is modern 2011 technology and about as powerful as a laptop can get. I am running a 2-month-old laptop with Windows 7. Let me assure you that this is not always the case. I have seen a lot of people completely dismissing this phenomenon as novice users with bad hardware and slow ISPs.