Just taking sick birds to the vet over and over is likely going to be very expensive and is not likely to solve the problem. Having the next one that dies sent for necropcy will likely tell you what you are actually dealing with. At least then you would know how to best treat or what the options are.
Also, just personal opinion, but reaching antibiotics for chronically sick birds opens things up to breading antibiotic resistant bacteria which make the problems even harder to deal with and do raise the posibility of getting a nasty campylobactor infection from eggs/meat ect. Just keeping birds around that have activ respritory infections and arn't getting better or staying appearingly healthy puts the rest of your flock at risk.
Medicated bedding and vitamins though they may be quite healthy arn't going to solve the problems either.