I have the idea that youtube gives ISPs tools (or that the ISPs get their own) to 'manage' the download. For example on my internet connection youtube never buffers ahead to the end of the video as it once did. One possible method would be to allows certain ISPs to use certain servers.
So I speculate that the problem might be connected with your ISP.
For a while I used a youtube management browser add-on which seemed to help.
SmartVideo For YouTube
These sound reasonable settings.
"under buffering options, ensure that the videos buffer on start, will not play until a certain amount is buffered and buffers while paused.
DO NOT check the first box that says to play the video on start"
Usual caveats about third party software apply.
Other people have suggested blocking certain IP addresses using the windows firewall.
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Here are some notes I made a while back. Sorry, I don't have the energy to ferret around further at present.
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http://productforums.google.com/foru...26-50-false%5D
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google [youtube tools for isps]
http://www.nanog.org/meetings/nanog5...Youtube.18.pdf
Dec 2011
This has info on youtube video server selection algorithm.
Flash mentioned, not html5.
"ISP3 has access to exclusive
servers even for non-popular
videos"
pytomo - Tomography tool working as a YouTube crawler - Google Project Hosting