When burning a finshed product from the timeline, what causes the picture to freeze for a second or two during various parts when the dvd is being played in a dvd player?

Not sure of the technical details but methinks it holds a frame near the end of a file while it seeks out the next file. You can overcome this by inserting a second or more of black video at the beginning and end of each title. It will still freeze, but no one will really see it.

You want the black video at the beginning because not all DVD players will be ready to play the first frame in a file. One second is long enough for most players to get their poop in a group.

One known issue is, when using dual layer DVDs, the transition from one layer to the other. Even commercial (aka pressed, or Hollywood) DVDs have this problem, some of come with a written warning on the box.

Another one: if the authoring software does not order the video files correctly (larger files are broken up), the player will need time to find the next segment.