Hi,
For the past several thousand pages, I've been having intermittent print quality issues with my M601DN. I've printed cleaning pages, cleaned the rollers, replaced the toner, to no avail. Right now I'm using a new, genuine HP toner (CE390A). We've also visually inspected the fuser, and found no obvious problems.
Intermittently, when I print a document, there will be a vertical white gap in the center of the printed text, and about 3.75" lower will be the text that should have been printed. This problem will typically continue for about 5-10 pages of the printout, and does not occur on all printouts.
The printer's about 2 years old (May 2012), and has been through about 28,000 pages of text.