Streaming throttled?

[#1]

I've been using YouTube and Iplayer recently and noticed some strange speed readings using the network meter. I'm synced at 7.5Mb and when i first start to stream I get a throughput of 700kB/s (basically max speed of the line), however after after about 3 seconds the speed drops to 100kB/s and stays that way.
I have 2 plusnet contracts on the Extra service and I am seeing the same thing on both lines, they both eventually drop to a download speed of 100kB/s. One line is in Leeds the other up in Newcastle.
Is this thorttling?  If so why is it still happening at 1am in the morning? I have only used 4GB of my 60GB allowance.
This seems to have only recently happened before I was streaming fine and had no speed issues.
Downloading zip files I can download at a constant 700kB/s so it isn't a line issue.
any insight anyone?

[#2]

That isn't Plusnet throttling, it's the way the server delivers the video to manage loading. The initial burst gets playing started, then as long as delivery keeps ahead of the viewing point the server can deliver at a slower speed. On Youtube the feed usually completes in 70% of the clip duration.

[#3]

It also accounts for a lot of people aborting before watching the whole clip - if the whole thing was downloaded at line speed the servers would be delivering a lot of data that was never watched (especially for people on fast connections). By managing it the way it does the user experience is fine and the amount of over delivery is minimised if people abort watching.