Have a token that you pass as people take their turn. A speaking stick as it were. This helps both the shy players and inhibits the talkers and those who can't wait their turn.
Talking over someone when it is their turn is just impolite, and while games can get exciting, if it gets to be a problem to the extent that you need to post to ask for solutions, then inhibit it all with the speaking stick.
People who don't pay attention? Draw them aside after the game session and ask them if they are feeling well. If they are? Next session, if they don't pay attention, keep going, and don't repeat for that individual. They will keep up!
It is part of the sad but true of GMing: If you have an hour and six players, each player only gets ten minutes.