So I spent like 5-10 minutes trying to get a video to stream by reloading and changing resolutions... nothing.. I tether computer to my Verizon 4G phone and BOOM, HD streams across no problems.. It is DEFINITELY a FIOS issue (Maybe they need to ditch alter?).
Did you try any of the fixes mentioned in this thread. I did and youtube now streams great on my laptop and on my roku box. All in wonderful HD with no buffering.