Big Country;
I would key in on the bare shafts "fly fine" but shafts with feathers go left. If "fly fine" means the bare shafts go where you are looking then it says everything is working right, but the addtition of feathers make the arrow go left.
I agree with Don S and suggest you work with the arrows. Try more point weight, an insert or longer arrows. If they eventually go where you look, or even start to go to the right, then you will know it is a spine issue.
The key distinction is "bare shafts fly fine, feathered shafts fly to the left".What do feathers do that could cause the difference? It would seem the two possibilities are;
1)feathers increase the spine,
2)feathers are causing some type of interference.
I hope you can work this out.
Jeff