What is happening is the tank empties completely when it should not. The air in the tank is what provides the pressure. Pumps use some of the pressure to work and when it drops to zero, it works less efficiently until the pressure returns.
Note that you need to calibrate your air pressure gauge with your water pressure gauge. The air gauge is used to set the precharge 2 or 3 PSI below the pump's kick-in pressure and most people use the water pressure gauge to determine that, hence the need to calibrate.