I just had all new 1.6 gallon toilets installed in my house to replace the 6 gallon size. One of these new toilets is "flushing itself" when ever someone sits on it. The flapper is new since it was just installed, so that shouldn't need replaced. Could it have something to do with the water pressure coming into the toilet, or would it be the toilet is not sitting level? The toilet is sitting on ceramic tiles. Any suggestions?