Print quality fades after a few hundred pages (90% remaining toner capacity)

[SanPabloTech]

HP LaserJet m608dn, 3 years old: print quality degrades to very light print after only a fraction (10%) of the toner is consumed.  Have installed a fresh Maintenance Kit ($400+, with new Fuser) and two fresh toner cartridges (37Y, rated for 40,000 pages) --both original HP brand-- shaken the toner cartridge as recommended, set toner density at Max value, and run the calibration/cleaning pages, but quality remains awful (severely faded).  What next?

[Valsimot]

When you install a new toner cartridge the print quality is good and then decreases or not?

What about the environment, dust, and smoke can cause dirty laser optics that result in poor print quality.