Have you noticed a bad odor from the affected birds? Coryza is one of the forever respiratory diseases, and is usually brought in by buying chicks and older birds from other people. It makes survivors and all others carriers for life. You may be able to close your flock and manage outbreaks, but can never sell birds or hatching eggs. I would definitely get testing done on a few birds, to get a real diagnosis. It may be hard to tell MG from coryza, and MG can be transmitted vertically through hatching eggs.