I have the same problem, it actually happens after the runs (about 30 to 45 min). I took gels, drank smart water, took elecotrolyte tablets, drank a ton of gatorade, it didn't seem to help. The only thing that seemed to help a bit was colder weather, I sweat more than anyone I know
Maybe you are doing your long runs at too hard of a pace, I think that was my problem but I have decided to scrap the long runs for now, the cramps were just too painful.
Also, do you stretch enough, I think people with less flexibility tend to have these problems more often.