is the unit running Intel Atom or Celeron? both processor class are not recommended for its performance issues. what you can try is to recode the video to use the following codec setting:
Video: h.264, 24fps, 4mbps, 1920x1080 max resolution
Audio: acc, 44khz, 176kbps
You can use the application called Freemake Video Converter from:
http://www.freemake.com/free_video_converter/ and recode the videos. if the issue persist, we need to know more about the processor model used.
best regards
-ADMIN