my friend who strings my racquet does a fairly good job at the begining but after maybe 4-6 hours of play, the middle mains seem to loose tension. what could the problem be? is it is nots, consistency on tensions, or bad batch of strings, or my form and hitting zone? (by the way i am using pro supex big ace 1.22) any relations with this problem or help besides findiing a different stringer. this is one reason for me to get my own stringing machine, the other is the price that pro shops give for labor.