Yes replace the developer housing and developer. Also replace the black toner and vacuum the black toner sytem as you could have debris or clumped toner traveling into the housing.
Thanks for the reply.
Shall I assume that the only way to fix the problem is to replace the component?
By the way, the spots in the image are like drops of solid toner, like the toner distribution is messing up. I'm not sure what DCD means. It might be the same thing. Just thought I'd mention it.
It looks like a black drum or developer housing issue.
Im going to say developer housing beacause of the (dcd) debris center deletion spots in your example.
Printer model: Phaser 7800GX
Cleaned LED window.
Changed black toner.
Changed Imaging Unit.