It happens to me too. At work or home. Win XP (home) Win7 (work). Explorer, Google Chrome, Firefox, all the same. What works for me is when it stops, usually about a minute or so into an average music vid is slide the button thingy to the end for a couple seconds then push it back to a few seconds before the stop point and it usually will download normally. You will see download filling in as it should ahead of the thingy.
It must be a youtube issue.
A man should look for what is, and not for what he thinks should be - Albert Einstein