My player does exactly the same thing.. It's because Sunplus firmware is poorly made. No matter what quality/frame rate/bitrate/container you choose, after 30 minutes of video playback, the player will start pausing for a second every 5 seconds.
http://www.mympxplayer.org/help-sunp...ge-vt9559.html
I investigate this a bit and it seems to be a memory leak somewhere in the firmware. I couldn't make mine to work, but I did a trick. You can use FFMpeg to split your videos in 2 or 3 pieces and so it will work.