I had a chance to speak with Steve Brown in customer service. He showed me a picture/image that you provided to us for review. He also let me know that he is currently working with you through the phone and email to resolve the issue.
The rings are definitely being caused by toner residue on the fuser roller. Have you tried getting rid of the dark black border on your design to see if the rings still print. As a test, I would try removing the black border or go to a less dense color to see if the black rings still appear. If we can confirm that the dark black border is the cause then it will help us to further trouble shoot the issue.
Looking at your design, I think that small border makes the design very challenging to align. In your case, the slightest movement of the sheet while feeding through the printer or on the in-feed would likely cause the labels to be out alignment. From a design perspective, I would highly recommend that you move away from having that border in your design. I think having that border will result in a lot more wasted sheets over time since there are variables outside of your control (e.g. slight movement in printer feed, etc.). If you are going to keep a border then I would make the border thicker and make sure that it bleeds at least 1/8" - 1/4" over the edge of the label. That will make it less obvious when there is some movement during the printer process. We obviously need to resolve the "ring" issue before we can focus on that border though.
Here is a link to a few articles that I think may be helpful.
This article shows how to create a bleed on your design:
http://www.onlinelabels.com/Support/Maestro/AlignmentGuide/Printer-Bleed.aspxThis article discusses best strategy for aligning label sheets when printing:
http://www.onlinelabels.com/Support/Maestro/AlignmentGuide/Design-Tolerance.aspxSteve Brown let me know that he is troubleshooting the issue with you and he is happy to send replacement sheets for further testing until we find a resolution that you are happy with.