Electrolytes my man ! People are afraid of salt like the plague when working out. Unless you are cutting for a contest or show or something, don't be afraid to add some salt, since clean healthy food doesn't have much sodium to begin with.
I have this problem as well, I sweat like a fucking man on trial. I also have really bad GI issues, so my doc suggested drinking mineral water on occasion (Evian, Pellegrino, Perrier, shit like that.) It has helped me alot, I have really bad cramping issues because of it.