Very familiar problem. It's a combination of heat stress and dehydration.

Long story short:

1. Work on your heat adaptation.
2. Drink plenty of water with electrolytes. Start drinking water with electrolytes, sports drinks or Pedialyte an hour before a ride.
3. Try pre-cooling and post-cooling methods. I use ice-packs on my neck immediately after a ride.

Here's a good recent article about conditioning for adaptation to heat on the Powertap blog.

A few nuggets:

• Heat acclimation develops quickly but "full adaptation often occurs after 10 to 14 days of exposure. Maximal sweat response improvements may take a month, and resistance to exertional heat illness may take twice that long."
  
• "A gradual increase in thermal stress is required to minimize the risks of prematurely causing exertional heat illness," and "research suggests that permitting a slight state of dehydration in conjunction with exercising in the heat may accelerate the rate of heat adaptation."
  
• Study participants from cooler climates who underwent successful methodical heat adaptation and improved performance in heat didn't enjoy any performance benefits back in their own cooler home country.
  
• Training indoors bundled up to raise core temperature may provide some of the same benefits, for folks who don't have access to training in hot climates in preparation for an event.

Years ago I worked for a freight company loading and unloading trucks on an open air dock. It wasn't bad except in summer. We weren't allowed to carry our own canteens and while there were water fountains on the dock we weren't supposed to use them. We got two brief breaks and one 30 minute meal break per shift. We weren't supposed to take any impromptu or as-needed breaks for water, to use the bathroom, etc. (Usually I'm a supporter of labor unions but this sort of rigidity to conform with the contract between labor and management did nobody any good. I wasn't a member of that union anyway so it wouldn't have mattered much if I'd ignored the rules.)

By the time meal break came around I couldn't eat. Then I had no energy for the second half of the shift. So I'd just bring fruit or candy since it was all I could eat and it gave me at least a little energy boost for an hour before the sugar rush backfired into hypoglycemia. I should have sneaked in a water bladder with electrolytes and a straw to sip on. Would have made that job a lot easier.

Living in Texas I'm accustomed to the heat. Heck, as an amateur boxer I used to do my roadwork at midday to acclimate myself for the autumn boxing tournaments, which were often in rings set up in outside parking lots, or in non-air conditioned gyms. Autumn in Texas is still stinkin' hot so it was good practice to work out in the same heat we'd be competing in.

And I always ignored the old timers who claimed we weren't supposed to drink water during workouts. They're full of crap. That kind of obsolete thinking got a lot of high school football players killed in Texas before the state sponsored a re-education program to retrain the coaches in scientific training methodology. If the trainers and coaches for Jack Johnson and Sugar Ray Robinson knew better their fighters might have won a couple of crucial bouts (Johnson against Jess Willard, Robinson against Joey Maxim). Both men faded badly from heat exhaustion late in their bouts and both got nailed by punches they'd have slipped or blocked when they were fresher. Nowadays you'll see ice packs used on the neck, shoulders and head between rounds to keep fighters cool, although the old school thinking persists that fighters shouldn't drink any water during a bout and usually only rinse and spit.

When the weather began heating up this spring I started riding midday at least once or twice a week to get myself re-acclimated. Often my only opportunities to ride are late afternoon and early evening, and in Texas summer that's still the hottest part of the day -- the temp doesn't begin to cool down until well after dark, and even then the pavement and buildings hold heat all night.

Anyway, study the article above. Don't get bogged down in the middle of the article where it goes into too much technical detail. Skip down a bit to the practical stuff.

I got careless working in the heat one summer about 15 years ago doing volunteer work for a favorite outdoor theater. First time I've ever experienced heat exhaustion. Apparently after working all day in the sun and not drinking enough water and electrolytes I passed out. Didn't fall, just slumped over onto a convenient bench or wherever I'd been sitting. Woke up to someone rubbing me down with ice and feeding me some fruit juice.

I was in my mid-40s then and realized I wasn't a young man who could ignore the warning signs from my body.

The past couple of summers I'd pre-hydrate with Gatorade or Powerade, but I can't stand the taste of that stuff. [MENTION=418438]TimothyH[/MENTION] mentioned NUUN tablets recently so I tried those several times. Seems to work great. I feel stronger during rides, still fresh afterward and my appetite is more than good -- I'm ravenous afterward. I ate entire large pizza by myself twice the past week after 20-50 mile rides in midday and afternoon heat. And I was hungry again within a couple of hours.

I'm also trying some DripDrop hydration powder, which seems to have the same ingredients as NUUN tablets. Cost is about the same, so there's no advantage over NUUN. But it's all Walgreen's had and it seems to work as well. The light lemon flavor was okay, though I'd prefer less sweetness. I don't really need any flavoring or sweetness, and it's okay with me if the electrolyte additives just have a slight tangy or carbonated sensation -- one I've tried tastes pretty similar to diluted Alka-Seltzer. Some folks might find that unpleasant but I liked it.

Also, you might carry some glucose tablets. Doesn't matter what kind, they all the same -- Glukos from the sporting goods store, the generic stuff sold for diabetics at the drug store. Chew one at the very first symptom of a bonk, even if you're not hungry. Heck, especially if you're not hungry. You should be hungry after a 30-50 mile ride, and during a longer ride. If you're not hungry, it's probably a warning sign. Yeah, I know, some folks like the gel packets, but those are expensive and probably won't do much for us average riders. I nibble on Kind bars throughout a long ride because they're affordable and not too sweet or sticky. And I'll chew a glucose tablet at the first warning sign of an impending bonk, although usually I can avoid this by eating and drinking properly beginning the day before a hard ride.