make sure you got the latest HD PVR firmware/drivers. make sure your integrated graphics or video card has the latest software/firmware/drivers. your computer may not be powerful enough to playback the HD video smoothly. you can try to record at a lower bitrate to see if that helps (try it on the lowest bitrate and see how the video plays). the video player you use to play the video back could also be the problem, try Media Player Classic, or VLC.
those suggestions should give you an idea of the problem.
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