Youtube streaming trouble

Hey, I've been having a lot of trouble with youtube streaming so I decided to finally look into it. In a nutshell this is one of those "Why is my internet so damned slow" problems. When streaming videos on youtube I tend to find that, despite having 10meg internet, I still find I can very rarely download as fast as I watch, so either I have to wait for quite a while or watch it stop and start. What I've found is that it tends to start off downloading at an alright speed and then slow down a lot.

To test it a bit more I've tried just downloading the flv manually in firefox, and I find a similar thing - 300kB/s or so at the beginning, and then a gradual decline in speed until by the time I've downloaded 10mB I'm down to 100kB/s. By comparison, using a multithreaded download manager it'll start off at the 1.2mB/s I should be getting and then gradually fall to about 400kB/s (obviously, still a lot faster).

So I guess I have a few questions:

1) Can somebody explain why multithreaded downloading is always loads faster?
2) What's causing my youtube videos to stream so much slower than the speed which I have the capacity for, and is there any way to stop this?
3) If not, is there any firefox extension or piece of software which would pull the flv from youtube and let me stream it using multithreaded downloading? (It would be nice if Miro did this, but it doesn't).

Big thanks for any help of any kind =)
Luke

[Setup: Running windows vista, behind a wired router]

EDIT: By the way, I'm talking specifically about youtube here. Other video streaming websites tend to work better