Boat Repair Questions? Ask a Marine Mechanic for Answers ASAP
I believe there is a service bulletin on that motor that requires the replacement of the 3 coils. There is no charge for this, even if the motor is out of warranty. Take it to your local Mercury dealer and they should be able to take care of this.
I'm not sure if this will solve your problem, but I certainly wouldn't go any further without doing this.