Maybe nero7 is still a little buggy
Can you try to do a burn with cd-dvd speed or with DVD Decrypter/ImgBurn? If with these other softwares you can burn to a lower speed, then is definitely a nero issue.
You can do it in a two-step process.
1) Use nero to create an ISO image of your burn (select Image Recorder as burning drive, so you can save an ISO file on your HD), and then
2) burn that ISO file with cd-dvd speed or decrypter/imgburn, selecting the right speed