"Tough As Nails": Calf Cramp

My left calf has severely cramped a half dozen times in the past four months--debilitating pain while running that is strong enough to force me to walk home. It is then sore as can be for many days.

I have been running 40 - 50 miles for over ten years--same healthy diet, same stretching regime, plenty of liquids and good shoes--New Balance 1223s or the equivalent. 

If anybody has any idea what I can do to alleviate this annoying issue please let me know.  Thanks.