Hey Dan - good and bad news I guess. Good you found it (and I'm pleased my eyes aren't trashed and I really was seeing a yellow issue :-) ). Bad that it will cost you some money.
Given it's intermittent, you probably don't need a head (if the head was failing, you wouldn't recover it) - it's most likely an ink supply issue so dampers, capping station, wiper blade all definites. Also consider the ink lines themselves (at least for yellow), and get them to check the I/C holder and the needles into the carts for damage or gunk. It would also be worthwhile doing a pressure test to ensure there are no leaks and that the pump is fine. I know it will cost more labour if you need to do a head later, but initial signs suggest that the head is fine. I'd at least talk to them about not doing the head and see what it would cost to do it later if needed (unless your head is many years old and you feel like it's worth putting a new one in anyway).
Also, use a new yellow once you replace parts - no point risking that there's anything wrong with the cart.
