Caching will not help your video if your internet download rate is slower than the video play. Your only option would be to download the video and watch it later "offline". I use
DownloadHelper for FireFox for saving videos.
If you do actually download videos to your hard drive and then still have jerky video, I'd suggest you try to use a different player as Windows Media player is not very good (I use
VLC).
OIC!