Hi Nick,
I had two pluckers, both have now stopped, how often do you spray your bird and when?

also try and keep the cage etc away from any chemical use including cleaning products and be careful what is on your hands when you handle him/her. the diet is good but try to add some mashed potatoes, scrambled eggs and rice.
Also try and get hold of some cod liver oil, most pet shops stock it.
one last thing, do you know what type of tree it is that he/she is climbing?

Have a look on the national parrot sanctuary (UK) website for list of what can be toxic to birds, they've got loads of stuff on there.
Good luck. KEEP US INFORMED.
MEL.