You have your Low Limit set lower or right at your running volume. Press and hold your volume/minute button for 5 seconds and a number will come up flashing. Lower this number and see if that cures it.

What I set mine at is about 25 psi application preasure or just enough to keep your check valves open. Press the volume/minute and slow down till the preasure drops to that point and read that number. Then press and hold again and put that number in. Then it will not let it go any lower that that when crossing a ditch or starting in corners.

you need to change this number everytime you change tip size. I just wrote them down for each set of tips and kept them with the machine.