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Need advice: 2010 Mercury 40 HP EFI 4-stroke ELPT4 outboard with 70 hours that looses power after running for more than one hour. Starts up quick, idles fine, and runs up to 6,000 RPM. (Motor matched to 13' Boston Whaler by manufacturer) After an hour of running a combination of 3,000 to 5,500 RPM, the engine WOT drops to 4,000 RPM, and no more. I turn the ignition key off and turn back on (starts up instantly) and it then will only run about 5,000 RPM WOT. I have drained 6.6 gal portable fuel tank and filled up with major brand 90 octane "R/V" gas and added "Sta-Bil 360". Replaced spark plugs and gapped plugs to specs. Replaced fuel line gas tank-to-engine, and bulb. (All connections are tight with new clamps). Replaced fuel filter. VST just serviced and water pump replaced by marine mechanic. Lower unit serviced, and motor oil/filter changed. Motor flushed after every use and "pees" great when running. Merc dealer checked motor by running motor on hose at idle for 15 min. Trouble not apparent until running for an hour at 5,000 RPM.