Several things - start by checking the tension on the drive belt and look at the drive sheave - if it is worn the belt will bottom out and usually the sharp v's will start cutting the drive belt. Check the hydro charge pump pressure then the hydro high pressure. Either will cause unit to stall out. IF high pressure is OK we have seen the heads "float" and the "milkers" relieve just before the relief valve and thus a large flow dump occurs. This is hyd checking 101. Really need SN to see what setup you have.