My Superdrive is not reading all of my DVD's... only some of them. For example, it will read the Incredible Hulk but not The Guardian. The drive spins, stops, spins, stops, then spits out the DVD. I've repaired permissions and have restarted but nothing works. Computer is refurbished, had it since Aug 2007, no longer under warranty. The drive is: Matshita DVD-R UJ-857. Any advice? I recently discovered that DVD-R's work well in this Mac but not DVD+R's. Thanks!