maybe it is to do with what you are eatng when? I run early morning, but don't get an afternoon slump - although as a teacher my classes finish at 3:30 so I have a natural 'break' there - if I am staying at school for a meeting then I get chance for a drink, walk around, or if I am going home then I get a break before driving home.
I eat brekki after running, have a snack around 10:45 - normall banana/apple and lunch at 12:45 (those times are dictated by the school timetable)