Why would I have water hammering at one faucet and NO others in my house?

I have purged air from my entire system, per protocol recommended by plumber. Also have recently replaced the bathtub fixture (where problem is). The hammering only occurs when I divert the water from tub spot to shower head. If I can get the shower head going, and then increase or reduce water flow, the water shuts down after the hammering and I have to divert back to tub spot to get it going again.