audio video out of sync with ntd38 novatron

hi guys i'm a newbie, i have an audio/video out of sync problem with HDMI novatron ntd38 multi-codec media player. Some downloaded HD movie files in .mkv format are playing fine but some movies' audio and video are out of sync. i press forward then return so that audio/video will sync. all downloaded movies are ok when i play it in my laptop using media player classic. does anyone has the same problem with Novatron ntd38 or its just the downloaded file? any new firmware to solve this problem? Im happy with the hardware, only this problem that makes me irritated. help! the only solution i got so far is to convert the mkv movie file to dvd file the audio was converted ok but it lost its resoluition from 1280x720 to 720x480.