Video lags audio by up to 20 seconds using Infuse on ATV2

I have my movies stored as MP4 files on a NAS drive plugged into the same 8 port hub as the Apple TV v2. Some movies consistently get more and more out of sync as the film progresses so that the audio can be 10-20 seconds adhead of the video. Sometimes I'm able to reduce the lag by exiting the film, and then restarting from the last viewed point but it never gets fully back into sync

The same films accessed from the same NAS drive across the wireless network appear flawlessly on a MacBook Pro using the Quicktime viewer with no sync issues at all.

I dont think it is network lag causing the issue and 20 seconds is a LOT of lag. Does anyone have any ideas as it's making watching these films intolerable.