+1 on the electrolytes.

I sweat like crazy, but if I hydrate well before and during, I won't be dehydrated afterwards. That could be anywhere from 1-2 liters an hour. What I will experience, very easily if I'm not careful, is mild but persistent hyponatremia.

Shootingstar, refresh my memory, but aren't you the same one who posted about trying to reduce your salt intake? Sounds like maybe you need to increase it...