I had a very similar set of problems with a Buff I rescued last year. I spent over $1500.00 taking her to local vets galore, getting xrays, drugs, and online vet advice, as well. She was diagnosed with coryza, impacted crop, worms, and on an on. I gave her gallymicin (sp?) and she would get better, then she would get worse. Finally, I contacted the University of Colorado vet diagnostic clinic. They are awesome. I couldn't bring her there as I live a state away, but they worked with me and basically determined that she had laid an egg internally, which was causing sepsis in her system.
She died, as the amount of antibiotics necessary would have been an IV drip for around 3 weeks. I shipped her to the vet diagnostic lab for a "chicken autopsy" and the diagnosis was confirmed. By the time any of this had been diagnosed she had been very sick for almost 4 months.
The vet, head of the avian department there, said that internally laid eggs are a real problem with the heavy laying breeds like buffs.
I don't mean to dump bad news on you. If this is what you have, and you've caught this early enough maybe you can get a heavy enough antibiotic to dose her clear it up without the IV treatment.
Good luck.