I have some suggestions:
1) Burn DVD-Video with DVDDecrypter instead of Nero. (or its successor, ImgBurn). You can set up DVDShrink to automatically launch DVDDecrypter and burn. Set the burn speed of DVDDecrypter at 4x. (It's not that Nero can't do it, it's just that it's almost impossible to screw it up this way with wrong settings, etc.) Use Nero for data discs, not DVD-Video.
2) Don't do straight copies. When copying a dvd+/-r, it's best to get it on the hard drive first. So use DVDDecrypter again, ISO Read/ISO Write. This also provides a useful check on the quality of the disc you're trying to copy. If DVDDecrypter can copy without throwing an error, you can be confident the source isn't the problem. Which leads to:
3) Use quality media. Anything made in Japan, any Verbatim. Look here:
http://www.digitalfaq.com/media/dvdmedia.htm
4) Since you have the Nero suite, open CD/DVD Speed, select your burner drive letter, and write down your burner's identifier string. While you're at it, insert one of the blanks you've been using, hit Disc Info, and write down the media I.D. (MID). Reference the MID to the list at No More Coasters, the link at 3) above.
5) Go to CDFreaks and find your burner using the identifier string.
http://club.cdfreaks.com/index.php?
Go to Optical Drives and the appropriate subforum for your burner. See if you can find newer firmware. Oftentimes newer firmware is released to provide better support for a wider variety of MIDs, also improved write strategies. Read up on what other folks say about particular firmware(s). Install ONLY firmware intended for your specific burner. Use the wrong firmware and the drive will probably be toast.
6) When you burn a blank, use the verify option, you may also want to immediately run a surface scan or disc quality test in Nero's CD/DVD Speed. Once you've got your method down and have been using it awhile with no problems, you may want to skip verify and scan. Your discretion, but for anything really important, I'd recommend doing the tests.
If you follow through on the above, your problems should be over. (Barring serious system problems or defective hardware, that is.) Good luck.
