Hello Jeffwat,
I am sorry to hear about your print quality issue with printing photos from your Officejet 8600 N911a model. I understand you are seeing lines between bursts. I have researched for a troubleshooting document for you to try, to help fix the issue.
Please click on the link below and follow each step in order:
If troubleshooting does not resolve the issue, as the end of the document states, please call HP's Technical Support. If you are within your warranty period, you may qualify for a replacement part or unit.
Good luck to you:)