It's funny that I am reading this right as I start to perform my "after lunch and pre commute home ritual".
This used to happen to me all the time, I'd get so sleepy I'd get cold and sweaty and feel weak as hell. Now, about half way between lunch and my 14 mile commute home I eat one or two peanut butter and jelly sammiches and drink a glass of fruit juice. The carbs in the jelly and fruit juice give me a boost of energy immediately and the protien in the peanut butter keeps me going in the long run. I still cary a snickers bar or a quaker oatmeal "trail mix" bar in my panier in case I feel a need for a refuel on my way.
Now I have a routine for my commute.
Morning before the commute - quaker trail mix bar and glass of OJ
Breakfast at work - Oatmeal and whole weat toast and coffee
Lunch at work - Usually one or two PB&J sammiches on whole wheat and water
Afternoon pre-commute - Same as lunch and a glass of juice

Haven't had the problem ever since