you’re using good media so that’s a start!
I’m not sure how well nero handles layer breaks. I know it used to have problems, but I’m not sure if that ahas been updated or not. I’ve never used Nero for DL burning so someone can jump in if I’m out of line, but ImgBurn seems to be the universal free choice with DL burns.
make sure all background processes are halted, remove any packetwriting software if you don’t use it (Nero InCD, Roxio DragtoDisk, etc, etc), make sure your burner firmware is up to date, and burn at a resaonable speed.
you can also set booktype in ImgBurn (if your burner allows booktyping/bitsetting)
keep in mind that the above is just general good practice and good sense when it comes to buurning dvds. booktyping MAY enhance compatibility, but it shouldn’t make much of a difference with newer drives so odn’t get too hung up on it if your drive doesn’t support booktyping… if you’ve done everything as suggested (update firmware, stop all unnecessary running processes, remove packet writing software, bitset to dvd-rom, and bnurn at the rated speed for the media) then I’m stumped.