"same shoes for months"... maybe it's time to get new shoes.
On the other hand I would wonder about dehydration and electrolyte levels, especially potassium. Most runners eat bananas but I'm a fan of dried apricots. Do you notice a difference in you calves during any time of day, like when you fist wake up? It just might be that you need a slower warm up. Or do they stay tight the whole run?
