I understand your pain (the heat not the headache). In Tulsa it has been miserable already this year, August is going to suck! Anyways, drink more. Get some Nuun tablets and put those in your bottles. I drink until I'm full during every ride right now. I got the same headache a few weeks ago and that is when I knew it was time to kick the hydration into another gear. Since then I have been good. On a 3 hour ride I will go through 4 - 24oz bottles (sometimes more) of water with Nuun tablets mixed in and will also eat at least 2-3 packets of Sports Beans.