Did this just start happening after replacing the power pack or did you replace the pack trying to fix the start and stall?
Can you give us more details and the history of the motor?
That extra detail helps a lot.
If the motor acts up with the black-yellow wire connected but runs fine when disconnected, it sounds like there is a short to ground in that wire.
When the key is turned to OFF or the safety lanyard is pulled, the black-yellow carrying the CD ignition voltage is grounded and the motor stops. In your case there may be a resistance reading to ground as it does not prevent the motor from starting.
Disconnect the Bl-Yl wire at the pack and at the key switch and use a quality meter set to high-ohms at one end and connect the other meter lead to the engine block for a ground contact. If there is any resistance reading, trace the length of wire to find the corroded spot or chafed area causing the problem. In some cases you may have to replace the wiring harness either in the boat or on the engine.
If the engine works with wiring disconnected, then there is a path to ground somewhere. You may need a tester called a megger to check the bl-yl wire as it carries around 350 volts or hgher at times.