Have you checked your bike fit? You may possibly be putting undue strain on your calves if the bottom of the pedal stroke is just a bit too low. If not, I'd probably play with lowering the saddle ~5-10mm and see what that does.
Also, what's your athletic history apart from cycling? Were you involved in other sports with high-intensity where your calves cramped often prior to max exertion? If so, there could possibly be an underdevelopment in the area proportional to your other systems.
Lastly, I'd look into stretching routines/flexibility. If after adjusting saddle height and you're sure the calves are fit for the work, then I'd also play with rollers on the calves for a couple weeks and see if that doesn't help.
I'm sure there's at least a couple more areas to look, but those would be the obvious places I'd address first.