I'm out of ideas, so lets have some fun and maybe put a little cash in your pocket. 1997 454 Mag MPI runs great at all speeds but will not idle and seems to be running rich at low speeds (less than 1500 rpm). Here is what I have tried so far;
spark plugs and wires, ignition module, IAC controller, thermostat (160), temp sender, temp switch, checked grounds, checked battery switch, compression test (140 to 150) all cylinders, no leak down done, TPS and MAP sensors, fuel injectors cleaned (not professionally), no vacuum leaks detected, fuel filter/seperator, VST tank screens and pump, diaphram in top of VST tank, fuel pressure regulator. That is all I can remember for now. I like a challenge but this is wearing me down. I will post this on the other 2 sites for those that have a preference. The winner will be publicly acknowledged unless otherwise directed. I have searched many forums for this problem as you can see by the things checked already. Help before there is a large pile of melted fiberglass in my yard.

Have you scanned the ECM??????? or swapped them with the other engine.
Are the spark plugs fouling????

My uneducated brain asks if you have checked base timing yet?

I always go for the very basics first. Fuel Pickup in tank line may stick during intial (low flow) idle. USCG valve may be sticking while boat is stationary? I doubt that's the issue as you've no doubt run the boat up on the trailer and it revs fine, but that's not under load so it may fall back to a flow issue.

I had a leak in an intake port gasket that gave me a similar issue once. No other way to check it but to remove the manifold and there it was!

Best of luck with it mate, I'm not smart enough to bother with fuel injectamation. I stick to carbs.

Sticking Idle air control motor, or possibly a small vaccum leak..

see my post on SOS...

sounds like a leaky injector.

I'm with Airpacker, a leaking injector could do it.

have you watched the timing at idle when it tries to load up ? is it backing off ? by now I would have installed a performer RPM air gap and a 750 holley...but thats me..

are any plugs wetter or more carboned then the others? that will tell you if its one cly over the others. did you also try cap and rotor? try to see if any plugs have a little oil on them this will also tell you if the intake is leaking from the block valley. the plugs are your eyes in the motor. how about some pictures of the plugs, maybe we can figure out what cly it is first or many. dan in my alcohol injected dragster we pull plugs every pass!

leaking riser gaskets @ idle?