This was previously discussed here: »Youtube at primetime issues
The issue does seem to be more of the CDN server, not throttling. I've noticed on YouTube it shows Canada next to the logo since the site design change. If you load the test video it streams perfectly but not the normal user videos. It's something on YouTube's end. I've also noticed that when loading a video they no longer buffer as much as before which could be one big reason they keep buffering while playing.
For me YouTube is hit and miss at any time of day right now. I can stream full 1080p for some videos then others max at 480p.