I have seen a couple of post about this issue but haven't seen a solution. My sprinklers start off with great pressure then after a couple of minutes the pressure drops. I am running a well with a 1HP pump, 15gpm tank. I have 6 heads on the zone. The pressure is 40/60. The tank sits at 60 with nothing running. When the sprinklers kick on it drops to 40 then after a few minutes it drops to 20 and stays there until the zone cuts off. When the zone cuts off the pressure jumps back to 40 and quickly returns to 60. I don't have any signs of a water line leak. What could be the issue?