HP Z2100 - Intermittent Black and Cyan Banding

I have started to get some vertical cyan and black lines on my prints. It is now happening more frequently and can range form a few fine lines to near solid patched as shown in the attached image.

I have changed the printheads but this has not solved the problem. Paper type and software makes no difference.

The lines appear to be about the same length as the printhead.

Diagnostic prints are fine.

Does anyone have any ideas what I can try next?