Help with Panasonic DMR ES-30V

I bought this recorder in order to copy a bunchg of old home movies onto DVD. On EVERY DVD that I make, the DVD has problems at around the 30 minute mark. It basically stops playing, although it will advance a little at time in a “choppy” manner. This keeps up until I finally hit “index” a few times on the remote and get it going again at around the 40 minute mark. What am I doing wrong?