The problem is the non return valve probably at the elbow end of the unit. If it is a new unit then i would say the rubber has not been installed and if it is an old unit the rubber has probably become dislodged

Water pressure makes the unit kick on and off. Most likely somehow water is draining back into the pump. It could be from the flapper on the check valve inside the discharge elbow or the flapper on the flush valve inside the toilet tank not sitting properly and allowing water to drain back into the pump creating enough water pressure that makes the unit kick on and off by itself.