I believe that's normal... It's just adjusting the ducer beam width that you see... If it's what I think you are describing then it's normal... You have the range set to auto?? Right??
I don't think he's referring to the steps you see under lake floor on occasion. He mentions stopping and starting.
I've seen the steps down under the bottom on the auto depth adjust but my guess is that he's talking about the screen not registering the bottom.