1. The lens could be out of alignment. If the player is still under warranty, I would see about getting the lens realigned.
2. You might have an older model that doesn't fare well with layer changes. If your movies are freezing in the middle somewhere, this can quite possibly be the problem. Many DVDs use dual-layering to increase storage capacity without requiring a second disc or to avoid recording info on the reverse side of the disc.
If the later ends up being your problem, it might be time for a new DVD player.