I have a Xerox Phaser 8400 with image problems.
After power up it prints a Startup Page - That page is black text which faded and slightly distorted in some areas and in areas that are in Color only...
What you are describing is a bad print head. You can try running the eliminate light stripes several times, and jet substitution, but most likely the print head will have to be replaced to remedy your problem.
You can try googling. Phaser printer print heads or phaser printheads.
There seems to be a secondary market appearing for those print heads. I can tell you from Xerox, if you have no maintenance agreement, they cost $750 per print head.
Honestly, having worked on about every type of color printer known to man, the new solid ink phaser printers, produce a dry ink image of 2400 x 2400 dpi with ink particles that align end to end. The resolution of those printers are remarkable and can be purchased for less than the price of one of those new print heads.
Now for reliable, albeit somewhat not as sharp a color image, but still exceptional, you really can not beat an HP.
The ink stick technology really no longer has the color vividness advantage it initially had. The inks that align end to end, have ended that advantage and are sure a lot cheaper to operate per page.