Is not an ISP ussue, I can watch any vod fine, or any other streaming site, but live channels buffer every 2 seconds, its been like that for a long time. 
Unfortunately this is a third party developer forum. You may reach out to help@twitch.tv or your ISP.
On a side note, I have had problems like this at times and switched to a 3rd party player that seemed to improve it. There was a network problem from my region into the Twitch CDN. It wasn’t Twitch’s doing, it was my ISP’s interconnect. The VODs do not play over the same CDN network, if I remember right because they are not encoded live, but I could be wrong.