There can be many things causing the changes your seeing, having an avian vet check would be very wise at this point, however two things that poped out to me in your post, one, the change of diet to mostly fruit, there is most definatly things laking with this type of diet only for a ringneck, and secondly, you say the bird is nesting, is she using a nest box? or a nest in the open?, if a nestbox I could see that might also be a problem with the lack of natual sunlight causing a vitamin D deficientcy, I hope the problem lies in either a nutritional or hormonal nature and not because of disease......good luck and keep us posted