In order of appearance:

1) Inexpensive DVD players

2) Layer change on RSDL (reverse spiral dual layer) DVDs - longer than 2 hours - that will usually happen around the 100-120 minute mark.

3) Finger prints on the outer area of the disc from handling - remember just like CDs, DVDs read from the center of the disc on out to the edge.

4) Dust on the laser lens - blow it off with an aerosol duster through the tray opening or remove the cover, blow it off and use a Q-tip "very lightly" dipped in alcohol to clean the lens itself.

If the discs are scratched, apply "good old-fashioned formula" Turtle Wax, let dry, rub off from the center on out, and then buff to a nice shine.

In my experience it is usually a scratched DVD, otherwise it might be an issue with the player itself. (rarely)