I have replaced the shower cut-off valve twice because when the tub faucet is on, the shower head leaks. When the shower is one, the faucet has some water running. What do I need to do?

"cut-off valve " do you mean the mixing valve or the stop on the supply line feeding the mixing valve. Sounds to me that you are talking about a shower/tub mixing valve that has a failiung diverter. Please be abit more clear in your descriptions.

Yes, it is the diverter.