Need Help Printing dark graphic banner shows off-color banding

It seems to me that the banding is not in all the surface printed. It seems that you have problems with the nozzles that are covered at some point. Clean heads with a powerfull cleaning

 

We've had that same issue, and no one could ever adequately explain what it was- or why it happens only every 8-18" or so (instead of every pass across), and only on the direction change. We changed thermistors, purged ink completely and tried new inks, replaced lines, heads... Nothing worked. The only thing we ever came up with to correct it was changing the print mode to a higher quality (which sucks, when doing banners or other "throw-away" graphics).