As Jim says, try media from a different package. I once bought a stack of Sony DVDs that turned out to be about 50% coasters. They were made in Malaysia. I have read that disks from Japan and Taiwan are likely to be better quality. My current fave: Verbatim -R, from Frys.
Also consider making a disk image in DVD-SP, and then use Disk Utility to burn that image to disks. You can set the burn speed, and tell it to verify the disk after burning. (don't know if these options are available inside DVD-SP, haven't done it that way in a while) Internet rumors suggest that using neither the fastest or slowest burn speed gives better results.
Plus if you have a disk image, you can take it to a different machine to see if that burner is any better.
And check your disks for scratches before and after burning. I've had disks that seem to come with scratches.
