I have a 350 mag mpi, 2003 and have been watching your problem . I had the same problem and was hoping that you solved yours . I bit the bullet and bought the Rinda Diacom program for my lap top. it showed no codes under load but over 2200rpms it would stumble and then shut off. it would restart with no problem, no codes.
I had changed the shift interrupt switch and the gear position switch, no change. I plugged in the old shift interrupt switch and let it just hang there. the test run went perfectly with no stumbling and not shutting off. I plugged in the installed shift interrupt switch and it stumbled and shut off, I re tested several times and got the same results .
I had changed the shift cables (both) and throttle cable some months back. what I have found is that the shift cables must be out of adjustment and ty wraped too tightly, just enough to give me a good shift but was causing the shift interrupt switch to activate just enough to cause the stumbling and then the computer would shut it down. Still no codes.
I haven't had time to readjust the cables but I'm confident that I found the problem.
Try jumping the shift interrupt switch with a jumper or a Bravo shift switch jumper.
Good luck, hope this helps
George
Without trying to highjack
silver_power's topic and we all await his latest testing's......I'll just mention in passing only that being you have an 2003 engine, your issues became known to Mercruiser and the repair for them was to replace the ECM....That mentioned, when I went looking for similar type issues you're having and while talking with Merc after the warranty timeframe expired, one of the service tech supports mentioned (described) what was happening was the ECM sensitivity was programmed to sensitively for reading (or sensing) the shift interrupt switch "vibration" while it was mounted on/to the engine mounted shift cable plate assembly.
One of his suggestions was to if possible, remove the shift plate assembly from the engine and try mounting it to the boats rear transom wall area as they seemed to have noticed removing the shift plate assembly from the engine to the transom area helped with this particular issue.
Pretty good you thought to try another switch just plugged in but not mounted and it seemed to help solve the problem....You must be very tech or engineered minded.
Your cable theory of being to tight somewhere along the lines seems very probable as it could be maybe vibrating the cable lines just enough activating the shift switch somehow.
While the ECM is connected to the shift interrupt switch and they talk to each other, I believe you'll notice that you won't be able to read on your marine scan tool hardware if the "shift" interrupt switch is being activated/deactivated causing issues with your engine running......The only switch you'll be able to read on the scan tool would be the "gear" indicator switch which reads if you're in or out of gear.... So in the following link, the scan tool will show/read item number 20 but not number 18,
http://www.mercruiserparts.com/bam/subassembly/31749/11652/300
Also bear in mind that there is a specific measurement that's supposed to ride between the switch plunger and the V of the shift plate Actuator lever.
If you don't find your tight cable theory to be the case then another test could be maybe you could try coming up with someway of adding something to help absorb possible vibrations to the whole shift plate assembly...Of course first unbolt the plate assembly from the engine and see if it helps any and go from there.
Page 3 of the following bulletin was basically the test you tried with a new switch not mounted,
Service Bulletin - Marine Mechanic
Hope this helps some, good luck.
