Here's a link on AGY (Avian Gastric Yeast)
http://www.finches.org/f-agy.mgi
AGY causes what most people would call "stomach" ulcers. You would be seeing watery stools, possibly a little blood in the stool. Your bird would be unable to get nutrition from the food and would start loosing weight. Additonally the birds droppings would start to smell.
AGY causes the bird to become anemic, due too the loss of blood thru the ulcers... I would think this would cause a reduction of activity in the bird, not hyperactivity.
However, it could explain the frequent eating and drinking, since the bird isn't getting the proper ammount of nutrition due to the ulcers.
According to the research AGY is nearly always a 2ndary infection over and above some other infection.
Generally I rely on my Avian vet for these kinds of diagnosis, however, I think we can resonably infer a few things:
Fact 1 - S76 treatment hasn't done anything
Fact 2 - Amoxitex was administered and you said the birds seemed to get a bit better
Fact 3 - Birds are still sick
Given the avalible facts as described, I'd venture the following educated
guess:
It's possible the birds started out with Airsac mites, S76 treatment was actually successful against the mites, but a 2ndary bacterial infection took over. Amoxitex treatment may have attacked the 2ndary bacterial infection, but revealed a tertiary infection, possibly AGY (yeast) infection.
In that case the Amoxitex is ineffective against yeast, and Amphotericin B is recommended (avablie as Megabac-S). If you have access to an Avian vet, you should def. take one of your ill birds in for a firm diagnosis, and they should have the appropriate medication on hand. If it is AGY you'll need to start treatment as soon as possible. The vet should be able to confirm AGY via a fecal smear (the yeast cells should be visible under microscope in the fecal matter).
Can you give us an update and more information?