Ira, glad I could help...I do think they're molting. Some of them will look really messy, and even have bald patches sometimes, while others molt slowly, so it's barely noticeable.
If you do find any mites or lice, I use Seven Dust (Or poultry dust from the farm store), powder them up real good, and then repeat in 10 days. It's a pain in the behind, because you have to powder everybody, and then you really need to gut the whole coop and put all fresh bedding inside.
From what you describe, since the new girls don't seem to be affected, I'd say most of your flock is in the process of molting.