Maybe the disks are the problem, or the burner. Nero really should fit the finished thing to disk. Unless it's corrupted in some way.

In nero startsmart under tools there is something called Nero CD-DVD Speed. Then burn a test disk and just follow the steps. It should then analyse the burner and the disk. I been a while since I tried it, but it should be pretty ABC if I remember correctly.

In case it's the disks, then try a brand called Verbatim. It's seems to work with everything.