jack russell terrier is scratching continuously

I have a 14-yr old female JRT/Pek. who favors the JRT look.
During a shedding season she has been scratching almost continuously (every
few minute intervals). She digs gobs of hair loose. We bath her about
monthly. Her regular vet checked her twice recently and couldn't identify the
cause. I suppose it isn't going to kill her, but any suggestions?