Mattias,
This is a little older engine than I am used to, but maybe your engine is running out of fuel. Boats have to lift the fuel up from the tank, so any air leaks on the suction side, a clogged fuel filter or a weak fuel pump could cause this. Above 3000 rpm, you're using gas faster than the pump can supply it, the carb runs dry, you stop. Cranking refills the carb and off you go. If your engine is fuel injected, get back to me.
Blacksmith