I'd Rather Be Lucky Than Good!
 

Obviously your first measurement of the fuel pressure with a guage thats off by about +4psi makes your fuel pressure readings somewhere about 43-44 psi which is exactly where they should be.
My thought here with no codes showing up when the power starts to disappear and rpms start falling is that possibly you have a fuel feed problem, possibly in the fuel tank, lines from tank to fuel pumps or possibly a clogging fuel filter or fuel water seperator that has to run a lot of fuel before it starts shutting off some fuel. You might possibly also have one of the fuel pumps either the lift (low pressure pump) or high pressure secondary fuel pump in the "Coolfuel" system that is getting reduced output when it gets hotter after long usage. You could also have some corrosion or rust or debris in the fuel rail and into the injector inlet screens that is building during use and then cutting of some fuel flow.
These are all just guesses based on your inputs and information here but unless you have some other electrical system problem like a dropping voltage I am not sure that your issue is in the ECM ,sensors , IAC motor or controls as these should all set codes if they are kicking in.
Hope you can find your gremlin as these types of intermittent problems are the hardest and sometimes most frustrating to find!
Good luck on your Sherlock Holmes Marine Investigator search.

Best Regards,
Ray @ Raylar

Thanks Ray for takeing the time to post ,i will check the fuels lines ,and change fuel filter ,i will cut open filter and have a look, check sceens on boost pump , then i will just have to keep going out in the boat and run it, keep local and wait for it to start breaking down (cant wait) when it did start to break down it was a small drop in rpm but got worse and worse ,it was just like some one was turning the motor on and off . Thanks again for all help , i will post when i find the prob. regards Sherlock

The first dozen times I had this problem it did not throw any codes, it did however finally throw a code that pointed to the IAC. If you got the scan tool, see if the technician can over ride the idle control...might help you eliminate this common problem.

i had a similar intermittent problem with mine. mine is a carb 454, but the concept will be the same. i found a cracked pickup in my fuel tank. it would run great while my tank was full (b/c i fill it every time i go to the lake), but then i'd run the tank down a bit and it would expose the crack and draw air into the fuel lines. when i was testing fuel pressure at the house, it would test fine (b/c the tank was full), and i think id have it fixed, then run it down the lake and get stranded again. yours might be similar, except that your crack might be further down your pickup and allows you to run down your tank further before you draw air. you didn't have the problem this last time our possibly b/c you filled your tank before leaving. on your long run, you probably ran down your tank really far and exposed the problem.

Thanks guys , been busy at work , but i am going to the boat tomorow and have a good look at the fuel lines, filter ,tank pickups and any thing i can think of. will post wot i find. thanks again ,

Hi all, had a good look inside the fuel tank ,and it was spotless ,no water, just like new ,both pickups were fine ,fuel filter was allso fine cut it open , checked all sensor plugs and as far as i can tell all look good and tight,so i will now just have to keep running the boat untill it starts to break down again i will leve the fuel presure G hooked upto see if presure drops when the breakdown happens. regards paul

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Hi all , i ran the the boat 3 times now the last time yesterday for over 2hours and its run great all 3 times , took it to max rpm and all good , i guess the faireys came and fixed it when i was:nt looking . thanks for any help that was posted . regards paul

i had this happen to me in my houseboat on lakepowel merc
v6's dts with alpha 1's. Both motors would run and after about 20 mins stbd motor tach would start to jump and eventually the motor would die, happened to me in a storm to...! any ways when the mechanic got to see it, it turned out 2 cylinders had baaaad compression.... Boat is a time share and port motor had been rebuild the year before for showing similair symptoms.