Youtube has had problems with this for a number of months, I even get TC's where the speed test is pegging 50+ but youtube is buffering. I have never found a decent technical answer,but someone has posted a solution that you can try and see if it works for you.
First I would run speedtest.net and make sure you are not having an issue followed up by a ping of about 500 packets to your provider website, IE
if its TWC then to TWC.net, etc... Not the commercial site the network side the page they host for you with the news and what not, you know the page that you ignore and use google news instead.
After that, the two saolutions I have noticed that have worked for people are
1. try a different DSN server set instead of your ISP's ie...