You didn't mention your internet connection type but slower bandwidth speeds do this frequently.. The best option is to stop the playback until enough data has downloaded to the machine so the video will play completely and smoothly.
Hope this helps.
I am running Vista and when I am watching a live video on Windows Media Player the video will run for approx. 1 min., stop, buffering, then freeze. I will cancel out and restart the video only to have it stop again. Could anyone offer a solution? Thank you