90 hp Force outboard RPMs surge in cool air

Motor runs fine around 5500rpms then will suddenly surge up into 6500-7500rpm range. i reduce throttle to bring rpms into safe range. after around a minute I have to increase throttle. runs good for a few minutes then problem repeats It seems this only happens when the air temperature is below about 75 deg. Manually mixed gas/oil. When air warms up will run at a smooth 5500rpms all day long. this is a '98 Force 90 by Mercury. Has done this since it was new. My dealer died shortly after I bought it so have been pretty much left out in the cold on minor problems like this

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