It might be the discs, the drive or the format is incompatible with the player.
You could try playing them in your PC's DVD drive, if it skips and jumps it's likely to be one of the first 2. In that case you could try another brand of disc, but if the problem persists you may need to buy a new drive.
If it doesn't skip it's likely to be incompatibility, in which case you will need to re-encode. You could also use Gspot (or something similar) to compare the files (codec, etc).