HDTV files require much more CPU power than other video files. Your CPU is a little bit far from those reqirements, but you can try this:
if the file is in x264 compresion format, open the file, rightclick -> options -> filters and here you can find the filters used to decode the stream. If it's FFDShow video, then go to it's preferences and disable post-processing. If it's CoreAVC decoder, then open it's settings and select no deinterlacing and skip deblocking. Things should work smoother.
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