My OfficeJet 6978 suddenly began printing stripes in a blue areas. It happens with both copying (with the computer cable disconnected) and printing from computer. The diagnostic page has horizontal yellow stripes through test pattern #3 which is supposed to be all green. The printer has standard white paper and all new genuine HP cartridges. I have done all the steps: cleaned the printheads multiple times, aligned the printheads multiple times, hard restart (twice). Thinking it was a defective blue cartridge, I bought another. Same result. I am using a Mac desktop computer but it doesn't matter since it happens when I copy with the cable disconnected. I am out of ideas unless it's a printer failure.