Pop in did happen a lot during the first few levels. It seems to always happen during the Human ones, but not the others.
It's a real pain int he arse, and makes the game look really cheap IMO. I mean, really, if this has been in the game for a while, fix it! Maybe don't add bumpmapping to the old mans face, get rid of a few bits and pieces. Don't make the Master Cheifs head so shiny, but do something about it.