Inks playing hide and seek.

-has this been a chronic issue, only recently happening more frequently?
-anything out of the ordinary lately, like service-work or head crashes?

ink supply is usually the culprit, either air in the system or something inhibiting the flow.
You can always rule out electrical/printhead issue if you do a data-swap, but my best guesses would be: a near-empty ink cart, airbubbles in ink line or printhead, a bad damper/seal, cracked printhead manifold (plastic tip that connects printhead to damper), or the capping station not always making a proper seal (either ink build-up or warped).