Final update. I did switch to 16%, though that did not seem to make a different. They're all molting, too.
I took two to the lab. The first died of hepatitis. The second died of lymphoma, and the report said I should consider my entire flock as exposed to Marek's ad to vaccinate all incoming birds. I lost another bird in that time, too. So I'm down from 33 to 24 in a couple of months.
Any suggestions on good year-round hatcheries that vaccinate? And the most hardy heritage breeds? No hybrids. Over the years, I've had Buff Orpingtons, Barred Rocks, New Hampshire Reds, Americaunas, Black Australorps, and Golden Comets.
I think I liked the Barred Rocks, reds, and Orpingtons the best. Any other breed suggestions, though?