It’s probably a QPel (quarter pixel) problem. Many Dvix/Xvid players choke on it.
Get MediaInfo (free) and see what it says about QPel.
Format : MPEG-4 Visual
Format profile : Streaming Video@L1
Format settings, BVOP : Yes
[B]Format settings, QPel : No[/B][/I]
If you are creating your own, make sure to [B]not[/B] tick ‘Quarter Pixel’ in Profile.