Printer is printing colorful bars across pages

[tharpa]

I have a Xerox Phaser 6128MFP, and even if I print in black and white I am getting red and yellow bars in a pattern across all pages. It told me I was low on black toner, so I tried to get a refill of genuine Xerox, but they no longer make them. So I bought some ink from another company. It seems to work, but still the colorful bars exactly the same as before. The printer recognizes that it's not genuine Xerox though, as the window says Non-Xerox Toner.

The printer's probably about ten years old - it's not Xerox doing it intentionally to get me to buy a new printer, is it?