two things come to mind after reading your post. Not being there nor running the testing myself makes me offer suggestions for your mechanics to test.
One is to monitor the voltage on the 55volt circuit using a digital meter and looking for any variation just before or at the moment the motor quits. The Evinrude diagnostic program can also record the engine conditions while the motor is running and when it stops.
Another test is of the fuel system and to monitor the fuel pressures and for any air bubbles while the motor is running at ALL speeds and when the motor quits.
To determine if it may be a boat harness or control problem, temporarily hooking up a different control box and cable assembly will determine if the problem is at the motor or not.
There are various things that could make a motor stop suddenly, and troubleshooting is required to find out if it's an electrical or a fuel related situation.
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"There is never just one thing wrong with a boat"
--- Travis McGee
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The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. If you elect to do it yourself, be sure to use caution, common sense, and to observe all safety procedures in the vicinity of gasoline, moving engine parts, high temperature components, heavy items, and electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, correct tools, and have an understanding of how to proceed with a repair. If you are unsure or uncomfortable with a procedure, a situation or a technique, enlist the services of a factory trained technician.
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