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?

What's the brand name and model number of your shower valve? One handle or two knobs? How old is it? Back to you. Tom