Definitely sounds media problem to me.
Traxdata or DVDMaster is the way ahead, don't bother with verbatim, vastly overpriced for wot they are and not that great.
DVDDecrypter is okay for backing up PS2 dvd games but PrimoDVD or Stomp Record Now Max the best.
Try
HERE for Traxdata. There was a shortage recently and prices went sky high but seem to be back down to normal.
Don't waste any more money on dodgy dvd-r media. Always use Traxdata,DVDMaster or any dvd that uses Ritek Dark purple dye and you won't b wasting any more money.