I'm new to rural water systems. Yesterday our water pump started cycling on and off constantly. The pressure guage climbs to 40psi, then the pump shuts off. The guage immediatly falls and the pump comes on as it hits about 15psi, climbs back to 40psi then off again, etc. If I turn the pump off I have enough water pressure/volume for one toilet flush. Any suggestions on where to start troubleshooting?