I've had problems with skipping player movement back when I tried to play Quake 3 on a 56k modem. Things would look fine for a few seconds, then a few seconds later, skip forward about 20 feet. Then it would be fine for another second or two, and then I would be falling off an edge to my death.
Although I have yet to play Halo for mac online, my experience tells me it's most likely lag.