Ok I have a house on city water. It had a 1/2 hp shallow well jet pump, and a 20 gallon pressure tank. The pump shot craps so I replaced it. The old pump never had a check valve before the pump, so I put in a dual check as per code before the pump. When main pressure drops at peak usage times the pump is just to be supplimental. The new pump is a 1/2 hp flynt and walling shallow jet pump with a 30/50 pressure switch. My tank is good and air is set at 28lbs. What is happening is when it drops below 30psi the pump kicks in and struggles to get to 50lbs, and when it does the pump kicks off and the pressure immediatly drops to right above 30 until a 3rd fixture is turned on. Then with the third fixture running it drops below 30lbs and kicks the pump on and acts like it is short cycling. I even tried a new pressure tank and checked pressure with 2 seperate gauges, and still the same problem. Pressure switch is new, I even tried it without the check valve, like it was originally just for kicks, same symptom. Anyone have any ideas, or need more info, I'm at a loss.