i will state that i am no expert with nero, but i have a feeling that your problem is related to discs. for a long time, i was getting downgraded from 16x burning to 4x with no apparant reason. so i thought my drive was shot too, but then it ends up being that the discs that i was using was the real problem. every single BnP that I've gotten flys at 16x, with stnd silver discs. I was using a nice 80min KLONE, that must not have been rated for anything over 4x.
just one thing to think about, good luck!