most likely the flutter at idle is that you are maxing out the output of your pump. the flutter is the pressure relief valve bouncing as the pump struggles at max demand. Making sure all the clearances in the pump are good and tight, is the only thing I know that will help. But you may be asking more than your pump will deliver, as long as it has enough pressure at speed it should hold up just fine. Check the pressure in reverse, that is normally the max pressure the pump can build. Look at the stock specs you will see reverse is almost twice what the forward gears are.