We get a "sputtering" of water pressure when we turn on the shower. It flows, but not a strong steady stream. However, if you then turn on the faucet in the sink in the bathroom while the shower is going, the shower flow goes back to a steady stream. Sometimes it will go back to the "machine gun" like stream while still running. What's my problem?

Brand name and model# of the shower valve? How old? One handle or two knobs? Back to you, Tom