Peculiar Youtube issue with sound sync

I have uploaded a video for a client to youtube. It is an hour and a half long documentary. Soon after the 40 minute mark, the audio and video go out of sync when the video is being played back at 720p or 480p. The video and audio remain in sync for the entire duration if played back in 1080p or 360p. Do any of you have any idea what might be causing this issue or who might be a good expert to ask about it? Thanks.