Yeah.. my initial thoughts are that it's the IAC - which I think is pretty much an Idle Control Valve. I used to have issues wtih the one on my old Mk2 Golf GTI 8v! - great motor that - so I know the niggling problems these things can cause on fuel injected engines.
I'll have a look around on the forum to see what needs to be done, but anyone got any pics of where the IAC is located? Or tips on what I need to do with it? Is it just a case of cleaning the thing, by soaking it in some solvent such as white spirit, or gasolene?
I spoke to a few Mercruiser dealers down my way in Australia (Northern Beaches of Sydney if anyone's nearby

) and one of them seemed pretty sure it'd be the block pressure sensor. Not sure what gave him such certainty..
I'd prefer to check off the things I can rectify myself before taking it in, and coming out with a hefty old bill (if only us Aussies could get the same value of you guys do in the US!)
I recogn I'll check the IAC valve and if it's not that, will take her in for a hook-up to the computer
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2005 Four Winns Horizon 230 - Mercruiser 320hp 6.2L V8 Bravo