2 496 HO's
Motors run perfect. every once in a while port motor simply dies. will be running perfect and all of a sudden its dead. Shift to neutral, drop the throttle turn key fires right back up. Sometimes it dies for a few seconds and re-fires itself before I get a chance to slow down and restart. The other motor is flawless. Checked connections at the ignition switches, no problem, checked the ground to the motor nothing. Boat only has one safety kill switch so that rules that out by itself. Dealer said they could look at it but without a constant they are shooting in the dark. Some days it will run fine through four footers at speed. Some days it cuts out several times in light chop. Very strange and bothersome. Could the ECU's be an issue? I was thinking of swapping them and seeing what happens. Its quite strange and i'm sure a wire somewhere but have no idea where to look first.

Check all the terminals on the main engine connector and the ecm for corrosion. Also see if you can spread the pins on the main connector slightly so they are in there a little tighter.

Have you checked for fault codes and reviewed the Freezframe Memory? There could be some clues there, but sometimes this type of problem will not generate a fault code. If therere is no indication of what the problem is, then I would start by monitoring the switched side of the Main Power Relay. Or maybe swap the relay from the other engine.

That said, I had a customer with twin 496Mag HO's and the starboard engine did the exact same thing. Took me several diagnostic sessions before I located the problem. There was a high voltage leak (arcing at coil tower from under the plug wire boot) at the coil that was located directly under the ECM. The Electro Magnectic Interference from the arc so close to the ECM would disable the ECM for a second and the engine would almost die, then it would recover, and so on.

Dave