The first thing I would do would be to check some other discs and try ripping with DVDDcrypter as it will give you real time rip speed and inform you of errors. You can also see it slow down as it has trouble reading. You also could try Drivespeed and see if it will let you set your read speed to 10X if your drive just turns out to be unable to run at 12X.

http://www.cdspeed2000.com/go.php3?l...rivespeed.html