For me, I've discovered that my head must swell while I ride. I was suffering from headaches on long rides. One day I thought to feel for the tightness of my helmet. It was significantly tighter after 3 hours on the bike than it had been at first! When I loosened it, the headache went away! Hasn't been a problem since.

DH gets headaches if he doesn't eat enough. So, keep up the experimenting with food. Starting out with some protein and fat before the ride will give you the energy you are going to need late in the ride. (Cause it takes longer to digest).