Help! NEC5100A Cant read DVDR and DVDRWs anymore!!

Hi there I have a very weird problem. Since a few months ago, my Inspiron 8600 with NEC 5100A DVD+RW drive has stopped reading my recorded data dvds anymore. It used to read them fine but now cant!! It CAN still read CD's and it CAN also play proper Movie DVD's. It CAN still read a blank DVD+R or DVD+RW however if i try burning something it will stop towards the end. I upgraded to Firmware version 10FE but same problem. The clue seems to be that when i put any data recorded dvd (that used to work) in the drive, after about 20 seconds you can hear what sounds like a kind of loading/spinning then stopping, then loading/spinning then stopping repeatedly. I bought a DVD drive cleaner but that has not worked either! Help!! Thanks Rohun