the one way intermediate sprag in a 400 is its down fall. one way to insure it is a sprag is put it manual 2nd. this allows the front band to hold the same drum to the case as the sprag. this is done to give extra holding power when hauling. if it does not move when you do this there could be valve body issues but i doubt it. in 2nd gear forward,intermediate clutch are applied and one way holds. if it was the intermediate clutch you would have no third and if it was the forward clutch you would have no forward at all. try manual second and let me know