Does this just happen once in a movie, maybe about half way through? If so, it is the layer change, and it is by design.
To handle all the data that is crammed on a DVD, and to avoid making you turn it over, it is recorded with 2 layers readable from the same side. It requires a refocus of the laser to read the second layer, and the pause you see happens when the DVD player refocuses. Some DVD players have a buffer which absorbs this time delay so you don't see it.