Problem writting DVD's with the NEC ND-2510 dual layer

Using the latest version of Nero 6.3… backing up movie DVD’s the DVD’s will copy but the copy is no good. Nero reports a successful burn but when the DVD is played back on either the computer or DVD player the movie in most cases is unwatchable as it freezes and and the sound goes out of sink with the picture and the picture has squares all over in blocks of green and red. Occassionaly it will copy some DVD’s but they are not perfect the whole way through and will occassionaly freeze and the sound will cut off for a second or two.

I have also tried coping already copied DVD movies to rule out that it is the ripping software and the copies still come out usless.

Can anyone help as I have just brought my first DVD writter to backup my DVD collection and at the moment it is no use what so ever.

I have already tried using a selection of different makes of 4.7GB media.

Many thanks.