MP4 playback on iPad - Audio Sync Issues

[nannynick]

I have trouble with some MP4 files (not all) where by once they have been playing for a while, such as 30 mins the audio drifts out of sync. Stop the playback and scroll back a bit, then play and audio is in sync again.

Is this an MP4 file creation issue - one particularly bad file was once I created by combining several files using MP4Joiner. (don't suppose anyone knows of other software that will join several MP4 files together, Windows7 32 or 64 compatible)

It is a problem with the player the iPad uses - same issue is playing on the iPad screen, or streaming to AppleTV. Or is it something else?