I have the idea that youtube gives ISPs tools (or that the ISPs get their own) to 'manage' the download.
No, they don't ... really, they don't ....
You'll just have to believe me on that, its too horrendously complex to even begin explaining to you !
In a nutshell, what you are getting yourself confused with is packet shaping, contention ratios, FUPs and all the other tools in the ISPs arsenal that provide them with a commercially viable way to give you high speed broadband at home for a stupidly low price per month !
The reason I asked the OPs the question about ISP related was for a completely different problem based on something I know was happening earlier in the week (temporary fault) .....
For example on my internet connection youtube never buffers ahead to the end of the video as it once did.
If you had bothered spending 30 seconds on Google, you would have found out YouTube switched to MPEG-DASH at the end of last year and hence the complete video buffer is the thing of the past....the point of DASH is it only buffers as much as is necessary, not the whole lot !
YouTube use a variety of DASH profiles and codecs depending on the player they are targetting.... which is why, coming back to my original point to you ... you can assume YouTube are doing a pretty good job on delivering a quality user experience (they've got a solid network and legions of excellent programmers ... the likelihood of a significant issue affecting a significant number of users is becoming ever more unlikely with the increased maturity of the YouTube platform).
Therefore issues you are experiencing are likely to be down to either your ISP having temporary issues, or more likely if you're on a cheap connection, the commercials you have agreed with them surrounding the technical constraints placed on your internet connection.
Usual caveats about third party software apply.
Using third party software to try to "fix" youtube is likely to be a pointless waste of anyone's time.... might as well wave a dead chicken over your computer and see if that helps your youtube !
Other people have suggested blocking certain IP addresses using the windows firewall.
That's a very silly idea instead. Have you seen the size of the Google/YouTube network ?
Totally and utterly pointless, and that's before we consider how pointless it is from a purely technical point of view !