Rereading the 'Anatomy' thread (search for phrase, "slight discontinuity one eighth of the way across the pattern"), I reckon the vertical banding in the magenta is just the printer switching between odd and even pairs of nozzles, and is normal.
The horizontal band, hmm. Having searched for...
I'm getting horizontal banding on colour printouts, especially in the blue areas.
Here's an example, where I used the printer's copy function to copy a magazine cover.
Here's a scan of a nozzle check:
(This is actually the nozzle check as it stands now, as opposed to when the problem first...