maybe worth checking with a test light or multimeter you have a constant live to stop solenoid with ignition on as it sounds like you loosing the live feed when you let go of the key from the start position hence it will run when you carry on cranking the engine.
(you need to find a live 12v wire with the ignition in the on position and also make sure it stays 12v when you stop cranking the engine)
(you need to find a live 12v wire with the ignition in the on position and also make sure it stays 12v when you stop cranking the engine)