Cockatiel epidemic! Last bird sick, vet tests inconclusive

Our cockatiels have all died except one. This started months ago and at first we thought they died of age or natural causes. But then they kept dying one by one, some within a month some within days of each other. They live outside in South Florida, in a large gazebo, not clipped so they can fly around, eat normal cockatiel seeds, sometimes lettuce. We've tried different things and cleaning more often, checking their food, trimming plants that grow around them and it kept happening. They also get covered when it is cold. We got down to the last one and we saw the second to last die in front of us. It looked fine and was perched when it suddenly took several deep fast breaths (almost like hyperventilating) and fell off and died. We took the last one to the vet and it looked fine and the vet said it was healthy and he could not figure it out. Had blood tests etc...Now it's been almost a month and the bird has been quarantined inside and within a couple of weeks started acting sick. Now it looks like it could just die at any minute, he's been fluffed up, off balance when he walks, shaking/trembling almost like he's having a seizure. Then when he tries to fly he crashes and falls (we put him back in the large cage about a week ago because he was banging himself all over the temporary small cage). We don't really know what to do and it's a mystery. Help!

nobody? I thought this was supposed to be an emergency forum