No solutions for you, but I get like this, too. It has gotten a little better over time, but it does seem like once I reach that point, *bam* I'm done. I get rapid breathing and heartbeat, and feel like I can't quite catch my breathe. I will almost invariably get a migraine later in the day when this happens (although last time I drank huge amounts of water and electrolyte replacement powder and I didn't; not sure which of these things made the difference or if it was just a fluke).

I don't think I sweat as much as other people do. One of my coworkers says his wife suffers from similar problems (headaches and all) and he said he thinks she doesn't sweat enough, and therefore doesn't cool off enough. I do know that I typically run a bit hot...I'm the first person in a room to be warm (and I live in Phoenix...great, huh?). In the past I had all my blood levels checked and everything was fine, but I think I might go have it done again, just to be sure something's not off with me.

One thing I've been doing lately is to wet a bandana and tie it around my head. It dries in about half an hour where I live; I haven't stopped to rewet it on my hour rides yet, but as I do longer rides, I will. It really seems to help a lot (as does leaving the house with wet hair). By accident I ended up wetting the back of my shirt and shorts (my camelpack had a leak in it LOL) and that helped a lot, too.