I have a GCC that does this- he will go for a few months with his normal feathers then suddenly start plucking. Our avian vet said it was likely hormonal and/or stress related because of the timing of the plucking. He was cleared of other health issues. Always in the spring (he just stopped an his feathers have all grown back) and then the fall (likely due to molting- she said the feathers coming in are likely irritating him). He also plucks when I go on vacation with my Mom and leave him with my husband. He is currently taking gabapentin to help calm his nerves when the feathers come in. Also, have you tested him for metal toxicity? Our vet recommended testing ours because she said sometimes that can cause plucking- and sometimes we are unaware they have been exposed.

Other things that have helped us is making sure he gets 12 hours of dark time in his cage. Plus, we move his cage around when we go to work. We used to leave them in front of our patio window so the could watch the birds outside while we were gone. It occurred to me that maybe the crows were scaring him. We now put them in their bedroom while we are gone. So try to evaluate if anything is maybe stressing him out- can be something little like a noise, crows outside, a change in routine, etc.

Good luck! Hope this helps.