My video playback is unsynchronised HELP!!!!!!!!!!!!!

Whenever i go to watch a video in the store or in my account the video is perfect but the sound is glitchy and off by about 30 seconds i have no idea how to fix this it is only in the store or my account if i play them on my ipod it works fine only when i am on my computer it is screwed up and i know its not my computer because i can play videos on it just fine its only in itunes that it glitches out