Sick Chicken about one every month to 6 weeks - Slow moving sickness?

I hope someone can help me figure this out. We have a large flock of about 40 hens with one rooster. We have been "doing" the chicken thing for about 18 months. We have all ages of hens. Some we bought as adults
1-3 years of age, and also we bought some chicks from the farm store last spring and summer. We even adopted some old gals to balance out the flock. We have a bunch of diffy breeds - all the main egg layers types.

I have noticed a trend over the last 6-8 months. It seems to start off with just one hen standing still, she will take to just standing off by herself all puffy. She will still eat and drink but stays mostly to herself. Then, after just a couple days of that, she will be down, and start to have trouble walking and keeping her balance, wobbly, using wings to try and steady. Eating and drinking slows down, not much desire but also can't always get to the feeders, and other hens will step on her and peck at her. I should mention at this point her comb has started to go pale and droop a bit, but her eyes are clear and alert. Vent is clear and normal looking. I have noticed some of the hens have messy bottoms now and then, but it does not seem to be a constent issue, comes and goes with the rain I think. I do know they have fowl mites, and I use white powder - (DE) in the dirt and nest areas. Lots of fresh water, and food, and free range of our land. I do feed them back any dirty eggs cooked with the crushed up shells. They do get kitchen scraps as well. The hens are healthy, brite combs, lots of eating, and scratching and eggs. Then - out of the blue... just one will have this come over them, it seems to be about once a month or so - give or take. I will watch and hope - but in the end have to dispatch her. Oh - age and breed does not matter it has happened to americanas, RI, jersey giant, silver laced, young and old hens. The last one was 1 year old americana - she acted like her bottom was too heavy. I checked for egg bound, and didn't feel anything strange, her body felt normal.

any advise or ideas - I would love to hear from you. I bought some duramycin-10 but don't want to use it - I don't even know if it will help and I don't want this in the meat or eggs.

Thank you. Jen