Panic B .... I had exactly the same problem about a week ago but for the last three days it has been perfect so hopefully it will sort itself out in your case as well. It was b' annoying while it was present so I can sympathise. I came to the conclusion that it was caused by problems AOL were having ... teething problems whilst upgrading. HTH
It's been running slow for weeks, we can't all have dodgy computers all of a sudden & mine's fine on all other sites. Occasionally I find a video that runs ok but mostly it's annoying on youtube.
If you google 'Youtube running slow' you'll see lots of complaints.
When you start up the video, pause it, you will see a faded bar continue to run along the length of the bottom, wait until it's at the end, or at least a fair distance from where it's been paused, unpause it and it will play uninterrupted.