DVD Burner leaves circular line on some DVDs

I am getting a circular line on the burned side of my DVD-R media. It only happens on some discs and not all duplicated discs. Usually the discs that have this pattern on the back have playback problems as well. Has anyone seen this before or know what might be the cause of this? I attached a picture which will more clearly show what I am describing. I am using Verbatim 8x DVD-R blank media and I am burning at 4x the max speed of my Pioneer burner (superdrive of a Mac G5) I also have a 2 drive duplicator from Primera which also uses 4x Pioneer drives and also sometimes leaves this line on the back of the burned disc. Just wondering if anyone has some information on this. Thanks!

Joe

This is either due to not the best media, buffer/source drop outs or using different write methods (CAV vs ZLV)!