HP 2600n Print Quality Issues

HP offers a troubleshooting page here

In addition consider the follow 

The other thing you can do is send a blank page or 1 character on a sheet of paper to the printer and see if the defect is present.  If it is, then send a two page test of the same and after the first page comes out, quickly open the toner door and the second page will be lying on the black etb (Electrostatic Transfer Belt) and see if the defect is present on the unfused image click here.  If the defect is present it has to be one or more toners.  If the defect is not present, then the fuser may have an issue.  You need to also flip back the shutter of the toner cartridges that expose the drum and see if the lines are present on the drum as that would indicate the toner is bad.

If your toner cartridges are near the end of their duty cycles, it's likely that the waste hoppers on the colors that are appearing in the print defect are full, and toner is back-feeding onto the page.  If you raise the shutter on an offending cartridge, you will probably see toner on the drum that corresponds to the defect on your prints click here