I'm no expert, but my guess is that you are eating too much simple sugar for that ride and like someone else said, your body doesn't quite know what to do with the surplus.

For a ride of that length, I'll normally drink only water and I'll go through a pack of shot blocks (I like to eat a few every 30 minutes or so). I believe a bag of shot blocks is equivalent to 2 gels or so. Anything more than that, and I find it's too much for me. In fact, I *think* that this may be why I too was having mid-ride slumps last summer. It only happened to me on much longer rides (80 to 100 miles) but I would lose all oomph at about mile 65 or so only to get it back at mile 75. Looking back, I think I was eating too much simple sugar, too soon into the ride. I'll be curious to see if spacing it out better works for me this summer (we are only doing shorter rides this season...tops of about 50 miles). I'm learning what works for me as time goes on.

It's also worth pointing out that I start every ride with a good meal/breakfast of approximately 40% carb, 40% protien and 20% fat about a hour or 2 ahead. If I choose complex carbs, this will hold me well into the first hour of the ride. I follow each ride immediately with a 4:1 carb to protien ratio snack (usually a Cliff bar) which helps replentish my stores and keeps me from wanting to eat everything in site about 30 min later.