Several things you may not already have tried (but my apologies if you have):
1. Use high quality top-brand media (eg Sony, TDK) not bulk-buy "cheapies".
2. Check your drive-manufacturer's website for a firmware update.
3. Use "TAO" mode (track at once) instead of DAO if possible. DAO is really only necessary when you want to make a copy of an audio CD without the silent gap between each track. For data that's not an issue.
4. I'm not aware of any issues with nero 6 and SP2 (I say that because SP2 did indeed 'break' a number of third-party applications for which patches had to be released) but it wouldn't hurt to ensure you have the latest update of nero 6. The last (and final) update prior to the arrival nero 7 was v188.8.131.52 for the core "nero burning rom" component , here:
5. Alternative is to UPGRADE to nero 7 if you find the trial version eliminates the probem you're currently having. nero 7 trial here:
It's a large download (150Mb - approx 20 mins. over a 1Gbit b/band connection).
Finally, disabling overburn won't make any difference unless you actually make use of that feature -- it simply alters the data measurement scale on the interface, and tells nero when to trigger the "not enough space left on disc" warning when you create a compilation of files. But as an aside I would always advise against overburning in any event. Overburning is not within industry standards, and my data is too precious to cram more on a disc than it was designed for, whatever the burning "geeks" may say.