I have to agree with the early am rides. I started riding at 4 just so I can ride due to my work schedule and the weather and traffic that I don't want to deal with. It's great, granted it's still humid as all heck, but definately a lot cooler and virtually no traffic at all.
Do you drink enough early enough when you ride? I had to train myself to start drinking water about 1-20 minutes into the rides because I was loosing so much just working in the heat. That can be affecting how your body's reacting to the heat later on in your rides or even after your rides. And I double what liza has said about drinking h2o during the day.