If it's only your adductors which ever cramp, do you work them at the gym on an adductor machine? Do you squat? Coffee and alcohol don't have anything to do with it. Probably some signaling screwup in the nerve bundle that controls that muscle. Stretch when you get back, like this:
http://www.bikeforums.net/showthread...1#post15372967

If you never cramp on the bike, don't drink anything for 2 hours before you go out, then have a glass of water when you get back.