Perhaps you're staying well enough hydrated but your electrolytes are out of whack?  Salt depletion can sometimes contribute to cramping.  Also some researchers have theorized that cramping in races is due to muscle fatigue.  Perhaps working on hip and hamstring strength in a weight bearing environment (i.e. NOT the hamstring curl machine) would help?  The stronger your muscles, the less likely they are to fatigue in the later stages of a race and the less likely they are to have a Golgi tendon reflex-induced muscle cramp. 

It's a theory anyway --Janet

train smart, achieve your goals... runningstrong