Shreck, you need to be far more 'aggressive' with your hydration and electolytes, as you are clearly on the ragged edge. Drinking only every 15 minutes just doesn't cut it. As you get in better and better shape, your system will not be as taxed, and you will consume less, but you have to let your body tell you that is happening. I ride often in temps of 110+. Here are the 'rules' I've gradually developed for high heat:
A. Have at least two bottles of water available for every hour of riding
B. Never, ever, allow yourself to "ration" water to make it last
C. Sipping is okay when it's cooler... when it's hot, take big gulps.
D. Never stop drinking
E. Overdo the electrolytes! Whether you mix it in your water, or take capsules, keep up the intake. You should taste salt in your sweat, and see it wherever your sweat has dried on your kit. And it's not just salt - use a balanced electrolyte source.
F. As you noted, cold water on your head makes a
huge difference. So does squirting some down the back and front of your jersey. Carry enough water that you can do this as you ride, anytime you are feeling the heat.
G. In the heat, it's really easy to not eat; to stop taking in the calories. This is bad... keep up the calories. From your post, you are NOT eating enough, which will cause you to lag as you get into the ride. Take in 150-450 calories (people have widely different needs/tolerances) per hour. A gel is maybe 90 calories...
H. Don't hesitate to stop if you need to, but, if you handle things right, you won't need to, other than to refill your water.
I. Plan your route, know where you can refill, and carry enough to get you through the longest period without rationing.
J. It may not look cool, but a hydration pack, along with your bottles, makes it easy to both carry enough, and to constantly sip.
K. Use jumbo, insulated water bottles, like these:
http://www.camelbak.com/Sports-Recre...hill-25oz.aspx The extra size, and the insulation, both make a big difference.
Hydration and nutrition are huge factors in performance, whether you are racing, training, or out on a club ride.