I was about to suggest you look at the wires when you described just what mine sounded like a few years ago. I didn't catch it until I rounded a cam lobe. Glad you found it and I hope no more collateral damage.

I just found this. I went thru the same exact thing on my port motor back in July. (About the same time as this thread) Wires 2 and 4 crossed. The boat would run, would backfire and die sometimes at idle, and was weak under power, it would run good enough I didn't think it could be crossed wires. Glad you found your problem too.