ATV3 audio out of sync...

I have my ATV3 hooked up to my Denon receiver which, in turn, is sending the signal via HDMI to a Panasonic projector. The content is on my 2011 Mac Mini upstairs. I have converted several movies to .m4v format using the current version of Handbrake using the ATV3 preset. Some were originally .avi, some were .mkv.

Some of the movies are out of sync - the audio is not matching up with the video. On some, there is an occasional "chiping" noise. Strange thing is, when I play them directly on the Mac Mini through iTumes there are no problems.

I don't know where to begin to troubleshoot. Should I start with the wireless network (most movies strea alright), the receiver (ditto), or something else?

I can't determine a pattern.