Problem watching online videos?

i missed a couple of episodes of heroes so i found a site that had the episodes that i missed and the video and everything works fine but the video does not play good in full screen mode, it is all jerky and does not flow smoothly. I'm pretty sure that it is not my video card because divx videos play perfect and dvds also work. And i have high speed internet so slow internet is not the problem either. does anyone out there have\had similar problems and could tell me how to fix this???? It would be alot better 2 watch episodes in full screen than in youtube size videos. thank you in advance. =)