Sherbie started this habit up in the last few months and it wasn't until this last "purge" he did of his chest feathers that I noticed he had two feathers growing from the same follicle. It seems to be an isolated incident, but it can cause a lot of irritation from what I've been reading. Have you noticed any of these?
* By the way, I did the same thing as you and the CBC, giardia, and all the other tests came back negative. We even checked for a fungal infection in the feathers. Nothing conclusive. That was back in August. I couldn't afford to keep bringing him back to the vet.
I've just been trying to keep him distracted with only minor success. He seems to get really irritated when the feathers have mostly grown out and then he goes in to fury mode and yanks them all.
I was given benedryl for him but it really didn't help him.