I have twin 2008 Merc 5.0 MPIs. The last two times out on the water I've found the port engine died after coming off plane. Engine always starts right back up without incident and seems to run fine afterwards. I thought it was an exhaust flapper issue so I called my Merc certified mechanic (who I think is solid) and asked him if he thought it was flapper. He said he doubted it as he hardly ever sees that problem and that the boat would likely hydrolock if that was the case. He suggested the IAC may be starting to go bad. When I got back to the dock I took a video but unfortunately can't get it to attach successfully to this post. The vid is showed the engine running at idle and then I put it in neutral and gave it a little gas, at which point it almost dies and comes out of it after I give it a little more gas. When I back off back to neutral it almost dies.
So, does this sound like an IAC starting to go bad or could it be something else? Starboard engine runs completely fine. And again, once the port engine gets through this sluggish stage at low RPM it seems to run fine at higher RPM. It is just a low RPM issue.