I have also noticed a similar effect on Verizon FIOS, where some video services work perfectly well during the day, but around 5pm or so start getting incredibly slow to buffer. Whether this is intentional throttling I'm not qualified to say, but it makes me quite hesitant to use Verizon in the future.
Occum's razor would suggest the problem is likely with the video services themselves, and/or congestion from other people who are now home from work watching videos, than something malicious from Verizon.