I have a Nec 3520 cd/dvd drive. It burned 8x speed dvd-r in about 8.5mins [average time for an 8 speed disc ]. it has now decide to burn 8 speed discs in about 20 mins. more than double the time. Is my drive "knackered" ? At one point I was burning with nero and the burn procedure stopped and nero came up with a warning of "power calibration error"? Any ideas?