Doc, glad you got yours fixed. Since you have I would like to throw my issue out there I'm having with almost the same motor.

I recently purchased a pontoon with the 4.3 mercury v6 mercruiser I/O with the alpha one outdrive. Put it in the water this weekend with a new battery (old one we had to jump off the day before and 6 years old so I replaced it that day) and rode around strong for 45 minutes no problems and no stops. When I went to pull back in the slough and put it in idle it cut off. Everything was dead as a door nail, no radio, no horn, no power period. Was worried it was the alternator, but when I went to charge it I hit battery test and we were already at (13 in the good range). I know the previous owner had problems with the bilge not cutting off, horn not working, and radio issues, but had them fixed them prior to the sale. I charged the battery some anyway and the next morning cranked right up and ran like a top. It was holding a charge on the gauges. Does this sound more electrical or not necessarily?
Thanks