Trigger points. Look into them. Unless you are experiencing cramping in other muscles groups when you are not training which could be diet/hydration related Im willing to bet you have some nasty trigger points in your calfs. I know I do. Get a massage therapist to work on them or try yourself if you are cheap (like me!) [/quote]
This, especially near the insertion behind the knee.
And give more attention to the lower phase. I had a similar problem after an injury to my right hip. I can’t remember who recommended this, but I started doing 8x8x8, which is eight half reps in the bottom position, eight in the top, and then eight more in the bottom (that’s one set, I traditionally did about 5 sets three times a week). This way you’ve doubled the volume of work in the stretched position, providing an “active” stretch.
Focus on flexing your feet or “gripping” whatever your feet are pressing against for complete foot/calf activation. Stretching has been surprisingly effective for me as well. Push against a wall and slam that heel into the ground. Good luck.