Sounds like you have a leak you need to find. How much pipe do you have underground? House on a slab? Turn the water off to the water heater and eliminate any possibility of a hot water leak. If you have a valve outside the house before the main water line comes into the house you could turn that valve off and see if you still lose pressure. If you still lose pressure and the valve is workign you know the leak is between the valve and the pump.