I'm going to second, third, whatever the suggestion for warning about transitions. I've always done that with my son (5) because I figure it helps with all small children. He's very used to it and is generally very good about it. So in the mornings before school, he knows that once he's done everything he needs to except put on his shoes and jacket, he can watch tv. He also knows that every single morning we need to leave for the bus before Magic School Bus is over. He always gets a warning about how much time he has. He also knows, because I told him at the beginning of the year when he asked to watch tv before leaving, that if he's difficult about turning off the tv even though the show isn't over, I'll solve the problem by not allowing tv in the morning before school. So, lots of advance warning about transitions AND consequences for poor behavior that I know he can control.
Also, while it sounds like transitions are the trigger for your son, I know that my son and most kids I know this age (AS or NT) have a much tougher time holding it together when they're overtired or hungry. Stuff that my son with breeze through without a problem most of the time can become insurmountable for him when he's tired. Maybe think about those issues, too?