A. This could be caused by many things. The most common is with a Fluidmaster 400. This is a great valve but be sure to install the 1/4" tube per their instructions. Use the clip that comes with it and do not just stick it into the overflow tube or it will go on and off every so often by itself. If that isn't it, take some food coloring and place it in your tank. Wait a few hours and if some of the color has gone into the bowl then you probably need a new flapper.