I just did the Sevin yesterday and prior to that I had already lost the bulk of the birds.  So far since the cage change and added shavings, I've lost just the one.  In reading from one of the links that was provided by White Mountains Ranch, they display symptoms of Avian Encephalomyelitis, which sounds awful.  But reading the symptoms of that particular "disease", it sounds just like what happens to them.  And if that's the problem, I have no clue where it came from, how they got it, etc.  I will have to do a little more reading about it.  It might also explain why some seem perfectly fine and others aren't able to overcome it.  According to the web site, it appears those that live are immune to it.  I guess my job now is to determine if that's really what it is and figure out what to do for it.  I really hope it's nothing quite that serious, but those symptoms fit my situation to a tee.