For the past month now while I run water in the bathroom the toilet will start bubbling. If the shower is running it will back up the shower and drain very slowly, if at all. I have had my toilet overflow on me about six times, and have had the plumber snake out my main line twice now, what is my next step? Thank you eddajune

Check for vent clogs. I think you however would be better served by a more experienced plumber that can actually clear the clog