I have a WorkCentre 6605DN. It works fine most of the time, but when I try to print pages with lots of solid black (1 colour, not 4), the print comes out with a mottled pattern - what I could only describe as a 'mackerel' pattern. The prints get gradually paler, until the printer pauses for a moment (to refill the ink?), then comes back with stronger black (but still not full coverage). The paler parts don't seem to be properly fused to the paper, and the print has quite a rough texture. I've cleaned everything I can think of to clean in the printer, and updated to the latest firmware, with no joy. I've tried three different black toner carts, and the Imaging Unit, Fuser and Transfer Belt Unit are all running at at least 80%.
Any idea why this is happening, and/or how to fix it?