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Hi, I'll need a few more details.
Is the engine getting hot? You said it has to cool off. How long does it run before stopping? Is it stopping just when you go to shift or when in neutral or in gear?
Has anything been done to this engine lately?
How are you cleaning the plugs.
Can you do some basic checks and tests?
Couple things come to mind.
First is the shift switch. Look at the linkage and bracket where the shift cable connects to the engine. There is a shift switch. If this is out of adjustment or sticking then it will cause the engine to stall when putting in gear.
Have a helper in the boat, engine off. Disconnect the shift switch and put an ohm meter, a cheap $10 multimeter will work fine for this. And while your helper shifts with the control in the boat watch the ohm meter for open and close. While he shifts spin the prop to help engagement. See if the shift switch sticks.
The other thing to check is the throttle position sensor, tps. Look at the top port side of the engine. The TPS is connected to the throttle linkage there. See if the TPS has been moved. There will be indentations where the washers were originally.
Post back what you find.