Check all connections
Check all appliances that are connected to the rainwater feed from the pump.
One of the most common problems is if the outside garden tap has been left dripping. This causes the pressure in the pipework to drop which starts the pump. The pressure then builds up and the pump stops.
If it is a Direct Feed System i.e the pump is supplying directly to the toilet cisterns it is worth checking that the torbeck valves in the cisterns are also not dripping.
Did this solve the problem?